Study on Technical Parameters and Suitability of Platinum-Based Metallic Glasses for Jewellery Applications
Study on Technical Parameters and Suitability of Platinum-Based Metallic Glasses for Jewellery Applications
Testing a series of platinum-based alloys for novel designs
Jewellery-specific standardised tests as well as bulk metallic glass (BMG)-specific testing methods were performed on a series of platinum-based BMGs with and without phosphorus, to evaluate their suitability as jewellery items. Their mechanical properties (elasticity, Young’s modulus and yield stress) were determined by three-point beam bending measurements. Hardness, wear and corrosion resistance were tested in comparison to state-of-the-art crystalline platinum-based jewellery alloys. The platinum-BMG alloys exhibit elastic elongation of about 2%. Compared to conventional crystalline platinum-alloys, their fracture strength of ca. 2 GPa and their hardness of ca. 450 HV1 is four and two times higher, respectively. However, the BMGs show less abrasion resistance in the pin-on-disc test than the conventional benchmark alloys due to adhesive wear and microcracking. Regarding the corrosion resistance in simulated body fluids, the BMG alloys reveal a slightly higher release of metals, while the tarnishing behaviour is comparable to the benchmark alloys. The phosphorus-free platinum-BMG alloy showed pronounced tarnishing during exposure to air at elevated temperature. The outstanding thermoplastic formability, a special feature of amorphous metals that can be crucial for enabling novel and filigree designs, was determined and quantified for all BMG alloys.
Platinum-based BMGs have been the subject of research since 2005 and possess high potential for jewellery applications (1, 2). One advantage is that they show low liquidus temperatures of about 520°C due to their chemical composition in the vicinity of the phosphorus-platinum eutectic composition (2, 3). The absence of microstructural features in amorphous metals results in a high as-cast hardness, high mechanical strength, an outstanding surface quality and the absence of volume shrinkage during solidification (3, 4, 5). However, amorphous solidification requires very high cooling rates in comparison to conventional casting in order to inhibit crystallisation. This leads to a challenging casting process and ultimately limits the casting dimensions. The BMG alloys Pt42.5Cu27Ni9.5P21 and Pt60Cu16Co2P22 show a critical cooling rate of around 20 K s–1 to successfully inhibit crystallisation and can be cast amorphously in diameters of about 20 mm (3), which is sufficient for their application as jewellery components.
Conventional platinum jewellery alloys challenge the casting process with their high melting temperatures of about 1850–2050°C and they reveal high shrinkage porosity (6, 7, 8) and low as-cast hardness. Binary jewellery alloys such as 95Pt-5Cu or 95Pt-5Ir possess as-cast hardness values less than 120 HV1 and ternary precipitation hardenable alloys such as 95Pt-2Ru-3Ga reach hardness values of about 200 HV1 (9). Work hardening, which is not applicable to investment cast parts, is required in order to reach a higher hardness (9).
Furthermore, BMGs show thermoplastic formability with high precision forming and enable new decorative designs (3, 10, 11). It has been shown that surface patterning of Pt57.5Cu14.7Ni5.3P22.5 is possible with an accuracy in the nanometre range which can result in a superficial hologram effect (11). Hence, the properties of platinum-based BMGs make them attractive for jewellery and watchmaking and have led to numerous investigations on BMGs with high fineness (85Pt and 95Pt) for almost 20 years. The focus was laid on their glass forming ability and thermoplastic processability, but their corrosion and tarnishing resistance as well as wear resistance are not yet as intensively investigated as a huge variety of conventional platinum-based alloys (12). So far, good scratch (13) and wear resistance (2, 14) are assumed for BMG alloys since they exhibit high hardness. For a better understanding of their applicability as jewellery items, this study focuses on technical aspects from alloy preparation to further manufacturing processes of alloys with a platinum content of around 85 wt%. A series of test procedures typically used for the characterisation of glassy metals in combination with standardised jewellery-specific tests was performed. The standard test procedures were carried out in comparison to conventional platinum jewellery alloys.
Mechanical properties such as elasticity limit, critical strains and wear resistance are in the scope of this work as well as the corrosion performance of the BMGs and benchmark alloys. Experiments in simulated body fluids and tarnishing tests for a few BMG alloys at elevated temperatures were conducted.
Since BMGs show the advantageous ability of plastic forming for novel designs (2), thermoplastic properties and the formability of the platinum-BMGs were characterised in this work to extend the understanding of their thermoplastic behaviour.
2. Materials and Methods
An overview of the investigated platinum-BMGs with their indications and compositions given in atomic percent (at%) are provided in Table I and weight percent (wt%) in Table II. The series of platinum BMG alloys chosen for this study consist of alloys from the platinum-copper-nickel/cobalt-phosphorus systems as well as nickel/cobalt and phosphorus-free systems. Throughout this work they will be named by their composition in atomic percent. Except the alloy Pt42.5Cu27Ni9.5P21, all BMGs reach a fineness of 85 wt% platinum. The conventional benchmark alloys and their compositions given in weight percent can be read in Table III. The benchmarks taken for the corrosion resistant tests are soft annealed. For the mechanical tests, they were taken in the as-cast and cold worked state. They are named with ‘a.c’ if they were tested in the as-cast state and are labelled with ‘c.w’ if the samples have undergone a cold work treatment. Table IV lists all alloys used in this work and the methods used on them.
The standardised tests were repeated three times for the glassy samples: once for the first alloy batch and twice for a second alloy batch. The conventional platinum-jewellery alloys were tested only once, since their properties are known and they serve in the comparative study as a reference.
|Alloy||Mechanic tests||Wear resistance and hardness||Nickel release||Artificial saliva||Annealing in air||Formability|
2.1 Sample Preparation
The alloys were prepared from high purity elements of at least 99.95% purity and master alloys. The master alloys for the glassy samples were produced following the two-step procedure described in (15). The samples were produced by arc-melting followed by suction casting. A custom-built arc-melting furnace with a suction casting inset was used. The alloy was melted in a high purity titanium-gettered argon atmosphere on top of a water-cooled copper crucible and then sucked into the mould cavity.
A second preparation method was performed using a modified MC15 tilt-casting device (INDUTHERM Erwärmungsanlagen GmbH, Germany). Here, the pure elements were inductively melted in a zirconia-coated alumina crucible under argon (0.6 Ar) atmosphere and consequently the melt was poured into a water-cooled copper mould. The master alloy Pt49.95Cu16.5Si6.4B24Ge3 was prepared by arc melting and subsequently cast in a centrifugal casting machine using a silica crucible in an atmosphere of 700 mbar argon. The level of impurities was expected to be similar to those given by the initial raw materials. The alloy composition was validated with continuous control of the masses involved in the alloying process. All samples were cast in plates with a thickness of 1–2 mm and a surface area of about 7–10 × 10 mm2.
The samples were ground using coarse to fine grained silicon carbide paper (500 to 1200 grains per inch) and subsequently polished with an alumina dispersion with 1 μm grain size. To verify amorphous structure X-ray diffraction analysis (XRD) with copper Kα-radiation was performed in a Vantec-500 diffractometer (Bruker Corp, USA). A general area detector diffraction system (GADDS) configuration was used.
2.2 Three-Point Beam Bending
Three-point flexural bending tests were carried out using an Autograph AGG testing machine (Shimadzu, Japan) to determine Young’s modulus, plastic limit and fracture strength. A three-point beam bending setup was chosen, which is used typically for metallic glasses. A more detailed description of the setup is given in (16). Due to the neutral plane in the middle of the beam, separating compressive and tensile strain, the propagation of a single shear band throughout the whole sample is hindered. This allows a certain amount of plasticity compared to a tension mode test (17). The cast samples were ground and polished resulting in a well-defined geometry with a cross-section of 0.6 ± 0.05 mm × 2 ± 0.05 mm. The samples are bent at a deflection rate of 0.3 mm min–1 with a probed length of 15 mm. Based on classical beam-bending theory, the recorded load-displacement information was subsequently transformed into engineering stress-strain curves (18).
2.3 Hardness and Wear Resistance
To guarantee the same surface conditions, the samples were ground and polished prior to hardness testing according to the procedure described in Section 2.1. The Vickers hardness (HV) was measured according to ISO 6507-1:2018 (19) using a KB10 instrument from KB Prüftechnik GmbH, Germany. The indentation was set by a force of 9.807 N (1 kg).
The abrasive resistance was examined with a tribometer pin-disc machine (CSEM, Switzerland). The tribological system consisted of the sample and a 6 mm diameter ceramic alumina ball acting as a pin. The normal force on the pin was 2 N, the number of rotations of the sample was 10,000 and the rotation speed was 500 rpm. Considering the diameter of the friction track of 2 mm, the sliding speed of the pin on the disc was about 52.4 mm s–1. Ambient conditions were 23°C and 50% RH. Afterwards, the wear marks were measured by profilometry, allowing a determination of the mark depth, and additionally investigated by scanning electron microscopy (SEM).
2.4 Corrosion Resistance
2.4.1 Metal Release in Simulated Body Fluids
For the metal release tests in simulated body fluids, samples of both BMG alloy batches were prepared as explained in Section 2.1 and the surface area in contact with the test solution was determined. The test solution for nickel release was prepared according to DIN EN 1811:2015-10 (20) and the alloys were immersed so that they were completely covered by solution. A second solution of artificial saliva was prepared according to ISO 10271:2020 (21) to test metal release. For both corrosion tests, the ratio of solution to sample surface amounted to 1 ml cm–2. The samples were stored in the solution for 7 days ± 2 h at 30 ± 2°C in a climatic chamber. Subsequently the released metal content was measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) according to DIN EN 15605:2010-12 (22). The tarnishing caused by the corrosive attack was investigated by colorimetric analysis according to standard test DIN 5033-1:2017-10 (23) before and after the test.
The samples’ colour is described by colour coordinates as follows: the luminescence [–L*, +L*] for black to white, [+a* –a*] for red or green appearance and [+b*, –b*] for yellow or blue. In the test daylight (D65) was used which was diffusely scattered by an integrating sphere on the surface of the sample. The measurements were performed with a 10° standard observer and a d/8 analysis geometry according to the standard test (23). The reflected light was split into its spectral components by a prism and these were then analysed by a photodiode array.
The Yellowness Index (YI) was determined according to standard ASTM D1925 (25).
2.4.2 Tarnishing in Air at Elevated Temperatures
The corrosion resistance was also evaluated by the investigation of tarnishing behaviour of the BMG alloys when exposed to air. Annealing experiments in air at elevated temperatures were performed for the phosphorus-free alloy Pt49.95Cu16.5Si6.4B24Ge3 containing silicon and boron and two platinum-BMGs Pt57.3Cu14.6P22.8Ni5.3 and Pt58.7Cu20.3Ag1P20. The exposure at higher temperatures accelerates the kinetics of microstructural processes, making potential changes detectable in a reasonable experimental time window.
The alloy Pt49.95Cu16.5Si6.4B24Ge3 was annealed at 501 K (228°C) which corresponds to a homologous temperature about TH = T/Tg = 0.88. To get an overview of the tarnishing behaviour of the platinum-BMGs, one candidate of each platinum-copper-silver/nickel-phosphorus alloy was chosen for the tests. Since the glass transition temperatures Tg of Pt57.3Cu14.6P22.8Ni5.3 and Pt58.7Cu20.3Ag1P20 differ by only 8 K, the average value was taken and to ensure the same homologous temperature of 0.88, the annealing temperature of 449 K (176°C) was determined for both alloys. During the exposure time, the colour change was frequently measured. Alloy Pt49.95Cu16.5Si6.4B24Ge3 and the alloys Pt57.3Cu14.6P22.8Ni5.3/Pt58.7Cu20.3Ag1P20 were annealed for 36 days at 501 K and at 449 K, respectively. The annealing tests of the Pt49.95Cu16.5Si6.4B24Ge3 alloy were performed twice with different samples to confirm the results. To increase the data accuracy shorter time intervals were chosen for the repeated measurement. Focused ion beam (FIB) was used to prepare samples imaged in SEM and chemical analysis was conducted by energy-dispersive X-ray spectroscopy (EDX).
2.5 Formability Tests
When metallic glasses are heated above their respective glass transition temperature, they can be thermoplastically formed in analogy to polymeric thermoplastics or conventional silicate glasses. The formability of an alloy is mainly defined by the temperature dependent viscosity, η, describing how well the material flows at a given temperature, in combination with the crystallisation time, tx, defining the time window of deformation until the material crystallises at this temperature. The formability, F, is defined as Equation (ii) (26):
The formability was determined for several platinum-phosphorus-based BMGs based on calorimetric measurements and thermomechanical measurements to determine the crystallisation and the temperature dependent viscosity, respectively. Further, thermophysical properties such as transition temperatures and enthalpies were determined by differential scanning calorimetry (DSC). A detailed description of the methodology used can be found in (27).
XRD measurements were performed on the samples cross-section prepared as described in Section 2.1. Figure 1 shows the patterns of all BMG alloys. No sharp peaks corresponding to crystalline peaks can be detected. Only two broad peaks are visible in the X-ray pattern, which is typical for amorphous structures. It should be noted that the small peak appearing in each diffractogram at the same position at around 55° is due to a fault in the detector. For the two alloys Pt49.95Cu16.5Si6.4B24Ge3 and Pt42.5Cu27Ni9.5P21 a structural pre-peak in front of the first sharp diffraction peak (FSDP) can be observed. For the latter this pre-peak is associated with an increased degree of medium-range order (MRO) (28). The observable shift of the FSDP in this alloy is probably associated with the lower content of large platinum atoms, leading to a smaller average interatomic distance.
3.1 Mechanical Properties
In Figure 2 the stress-strain curves of three nickel-free platinum-based BMG alloys obtained by three-point beam bending are displayed in comparison to those of three conventional crystalline platinum-alloys measured by tensile testing (reproduced from (9)). Each alloy composition reaches its elastic limit at 2% strain and with a strength of around 1.6 GPa (Rp2). Above 2% strain, the Pt58.7Cu20.3Ag1P20, Pt57.8Cu19.2Ag1P20.6B1.4 and Pt60Cu16Co2P22 alloys exhibit plastic deformation before critical failure. Pt60Cu16Co2P22 shows the highest plasticity and fractures at approximately 3.3% strain. The results of the three-point-beam bending experiments are provided in Table V. Maximal Rp2 values are detected for the Pt58.7Cu20.3Ag1P20 alloy. Compared to their high strength, the Young’s modulus of metallic glasses is relatively low, resulting in lower stiffness. In contrast to the platinum BMGs, the crystalline alloys demonstrate in the tensile tests significantly lower yield strengths (<500 MPa) and a pronounced plasticity.
|Alloy, at%||Elasticity, ɛ, %||Elongation at fracture, ɛF, %||Young’s-Modulus, E, GPa||Yield stress, Rp2, MPa|
|Pt58.7Cu20.2Ag1P20||2.1||2.5||88.8 ± 7.3||1721|
|Pt57.8Cu19.2Ag1P20.6B1.4||2.1||2.3||82.5 ± 3.7||1604|
|Pt60Cu16Co2P22||2.0||3.4||88.1 ± 4.1||1660|
aBenchmark tensile tests (9). Yield stress Rp0.2, MPa
3.2 Wear Resistance and Hardness
The results of the wear resistance tests are depicted in Figure 3. For the BMG alloys, the mean values and standard deviations of three test series are plotted. All diagrams present the values for the BMG (black) in comparison to the conventional alloys (red). In Figure 3(a) the hardness values, given in HV1, are shown and the overall hardness values of the BMG alloys (around 450 HV1) are more than twice as high as those of the crystalline work hardened alloys (around 250 HV1) and about four times higher compared to the crystalline alloys in their as-cast state (around 100 HV1). The mean values of the wear mark depth depicted in Figure 3(b) reveal slightly higher values for the BMG alloys than those in the crystalline benchmarks. The mark depths of Pt57.8Cu19.2Ag1P20.6B1.4 and Pt58.7Cu20.3Ag1P20 are about 1 μm larger than those of the nickel and cobalt bearing BMGs. Pt57.3Cu14.6Ni5.3P22.8 show the lowest wear mark depth. In Figure 4 the wear mark depth is plotted against the hardness and lower values for the cold worked alloys compared to the softer as-cast state are discernible. Comparing the platinum BMGs among themselves, no evident correlation between hardness and wear mark depth can be observed.
In Figure 5 details of the wear marks are shown for the BMG alloys Pt57.8Cu19.2Ag1P20.6B1.4 (Figure 5(a), 5(d) and 5(g)), Pt42.5Cu27Ni9.5P21 (Figure 5(b), 5(e) and 5(h)) and Pt49.95Cu16.5Si6.4B24Ge3 (Figure 5(c), 5(f) and 5(i)). In Figure 5 different magnifications and different degrees of detail are shown hierarchically from the first to the last row. The positions of the magnifications are marked by a square. The overview images of abrasive attack of the alloys Pt57.8Cu19.2Ag1P20.6B1.4 and Pt42.5Cu27Ni9.5P21 display wear marks with flat pressed material at their respective edges. The marks of Pt57.8Cu19.2Ag1P20.6B1.4 reveal peeled off particles and cracks in the border zone. In the detailed depiction of the mark centre of both alloys (Figure 5(d) and 5(e)) where the applied load is maximal, scores next to deformed and flattened material can be observed. In the magnification shown in Figure 5(g) and 5(h), the surface appears coarse and rough and daubed material can be detected. In contrast, the overview image of Pt49.95Cu16.5Si6.4B24Ge3 shows cracks in the centre of the marks (see Figure 5(f) and 5(i)) together with an accumulation of apparently loose particles next to the wear tracks. Since less daubed-like regions and mainly cracks can be observed, the abrasive damage looks remarkably different to the other BMG alloys. Comparably, the abrasive damage of the cold worked benchmark alloys 950PtW and 950PtCu observed in SEM is reported in Figure 6. The chosen magnification corresponds to those of Figure 5(d), 5(e) and 5(f). Smooth and plastically deformed surfaces are visible. The slightly harder 950PtW alloy also reveals partially removed material.
3.3 Corrosive Attack
3.3.1 Metal Release in Simulated Body Fluids
Because the potentially hazardous to health element nickel is alloyed in three platinum BMG systems, their metal release was tested twice by the standardised test (20) to quantify the release of nickel in aqueous solution. The results, listed in Table VI, show a release of nickel far below the limit given by the industrial standard for jewellery applications. European legislation allows very low nickel release rates (20). Therefore, although not forbidden, most European jewellery manufacturers refrain from using nickel in their alloys.
|Alloy, at%||Nickel release, μg cm–2 week–1||Limit value, μg cm–2 week–1|
As described in Section 2.4.1, the samples were stored in artificial saliva to determine the metal release. Metal release of the conventional benchmark alloys was only detectable for cobalt and copper. The results of element release for the BMGs and the benchmark alloys is plotted in Figure 7(a) and the value of copper-release normalised to the initial copper content is plotted in Figure 7(b). Mean values and standard deviations from all tested samples (Batches 1 and 2) are depicted.
The BMG alloys release mainly copper and phosphorus. Comparing the other BMG alloys, a higher element release was detected for Pt42.5Cu27Ni9.5P21. The element releases of the other BMGs are similar and about four to seven times higher than for the conventional alloys. The platinum release of the conventional alloys lies below the detection limit. Figure 7(b) displays the copper release of all alloys normalised to their initial weight percent of copper. The differences of normalised copper release between BMG and conventional alloys are lower due to higher copper content of the platinum BMG alloys.
As explained in Section 2.4.1, the change of colour caused by corrosive attack was determined. Figure 8 compares the overall change in colour between the BMG alloys and the crystalline benchmark alloys. The results reveal similar changes of colour of the BMG alloys and the conventional crystalline jewellery alloys.
3.3.2 Exposure to Air
Exposure tests were performed on samples of the alloys Pt57.3Cu14.6Ni5.3P22.8 and Pt58.7Cu20.3Ag1P20 as well as two samples of Pt49.95Cu16.5Si6.4B24Ge3 alloy from different batches. The samples of Pt49.95Cu16.5Si6.4B24Ge3 and Pt57.3Cu14.6Ni5.3P22.8 after the experiment are shown in Figure 9 and the calculated colour change of the platinum BMG alloys by exposure to air is shown in Figure 10. Macroscopically a pronounced colour change can be observed for Pt49.95Cu16.5Si6.4B24Ge3 whereas the Pt57.3Cu14.6Ni5.3P22.8 sample surface retains a silver metallic lustre. The colour change of the Pt49.95Cu16.5Si6.4B24Ge3 alloy composition, which is about seven times more pronounced than that of the Pt57.3Cu14.6Ni5.3P22.8 BMG, is clearly visible and appears brownish to the eye.
In Table VII the values for L*, a*, b*, YI and ΔE are given for the samples after annealing. To express the effect of tarnishing and to get an idea about the values and the corresponding lustre, the colour coordinates of an untreated 950PtCu alloy are given in comparison. Since this alloy was not annealed, no data after thermal treatment are given. Due to the brownness of the Pt49.95Cu16.5Si6.4B24Ge3 sample, the YI, which is commonly used to describe the lustre of gold and platinum-based jewellery, does not represent the colour here. However, it is given as additional information in Table VII. The a* and b* values of the Pt49.95Cu16.5Si6.4B24Ge3 samples differ slightly from those of the less tarnished samples and the benchmark reference. The luminescence L*, which describes the brightness, reveals pronounced differences for the tarnished samples. Conventional jewellery alloys reveal L* values of about 85 (29). Compared to the brightness of a conventional platinum-jewellery alloy, Pt49.95Cu16.5Si6.4B24Ge3 possesses less than half after tarnishing in air.
|Alloy||Exposure time, days||L* (D65)||a* (D65)||b* (D65)||YI (D1925)||ΔE|
Since the alloys Pt57.3Cu14.6Ni5.3P22.8 and Pt58.7Cu20.3Ag1P20 did not show any differences in their superficial appearance after the annealing experiments, only Pt49.95Cu16.5Si6.4B24Ge3 was examined in more detail. The SEM investigation of the surface after exposure to air is shown in Figure 11. The position of the cross-section prepared by FIB is shown in Figure 11(a). The SEM image of the FIB section is displayed in Figure 11(b). In addition to black boron-rich formations, which might not have been in solution before, small dendritic structures (marked with white arrow) are present. By using EDX, a carbon-rich phase was detected right underneath the surface with copper-oxide rich (dark) and a brighter platinum and silicon phase in the adjacent surrounding bulky matrix. The amount of carbon underneath the surface might originate from an organic platinum compound formed during sample preparation.
3.4 Thermoplastic Properties
Calorimetry was used to determine the characteristic temperatures of the glass transition such as glass transition temperature, Tg, and following crystallisation, Tx, together with the melting event, where solidus temperature, Ts, and liquidus temperature, Tl, are determined. The characteristic temperatures of all alloys are summarised in Table VIII. The glass transition temperatures are located within the small region between 501 K and 514 K. A larger spread can be seen in the onset of crystallisation, where the alloy Pt42.5Cu27Ni9.5P21 shows the highest Tx at 597 K and Pt57.8Cu19.2Ag1P20.6B1.4 shows the smallest value at 565 K. With respect to thermoplastic forming, the thermal stability of the supercooled liquid is of most interest. The thermal stability can be quantified by the width of the supercooled liquid region, ΔTx = Tx – Tg. Here, the alloy Pt42.5Cu27Ni9.5P21 shows the highest stability with ΔTx = 83 K. Nevertheless, this quantity is not a measure of the change in viscosity of the supercooled liquid, which is crucial for thermoplastic formability.
The isothermal formability, F, includes both thermal stability and viscosity. In Figure 12 the temperature dependent isothermal formability is shown as a function of inverse temperature, normalised to the glass transition temperature for 20 K min–1. Comparing the platinum BMG alloys among themselves, the systems based on platinum-copper-nickel/cobalt-phosphorus show better formability than the cobalt/nickel free platinum-copper-silver-(boron)-phosphorus alloys. The larger temperature range covered by Pt42.5Cu27Ni9.5P21 originates from its large thermal stability. The superior formability of the other two platinum-copper-nickel/cobalt-phosphorus alloys is due to their steeper decrease in viscosity around the glass transition, i.e. they are kinetically more fragile (30). Still the formability of all tested platinum-based alloys is much higher than those reported in (31) for other families of metallic glasses.
The investigation of mechanical properties by three-point beam bending reveals yield strength values Rp2 of about 1600 MPa for all tested platinum BMG alloys. Deformation is mainly elastic for the majority of the platinum BMGs, in accordance with the literature (32). The strength of a series of conventional platinum jewellery alloys with 95 wt% platinum was previously measured by tensile testing, commonly used for crystalline alloys (9). Although the testing methods used for the different material classes are not identical, the magnitudes of strength are still comparable with each other. Studies on crystalline TiAl6V4 using three-point beam bending (18) and tensile tests (33) reveal comparable results. For the majority of platinum jewellery alloys (9), yield strengths (Rp0.2) of about 150 MPa to 300 MPa were detected, as shown for 95Pt-3Ru-1.5Ga, 95Pt-5Ir and 95Pt-5Cu in Figure 2. The maximum Rp0.2 of about 421 MPa was reached for 95Pt-2Ru-3Ga. Hence, the platinum BMG alloys demonstrate about eight times higher yield strength values (with about 2% elasticity) than conventional platinum-alloys. Furthermore, the BMG alloys exhibit Vickers hardness values more than twice as high as those of the crystalline work hardened alloys.
The abrasive behaviour of BMGs was investigated. Against expectation (2, 14), their high hardness does not result in higher wear resistance. The BMGs display a more pronounced abrasive damage than the crystalline benchmark alloys. The conventional crystalline alloys show mechanisms such as microploughing and wedge formation which may occur during abrasive attack (34) and which are not expected to be accompanied by a loss of material. Proof for microploughing as damage mechanism was shown for 950PtIr in a previous study (12). In this work, a similar mechanism was observed for the 950PtCu alloy.
SEM images of the platinum-phosphorus-based BMG alloys suggest that worn-off material is plastically deformed and sticks to the surface. This observation points towards an adhesive wear mechanism, which has already been suggested for platinum-phosphorus-based BMGs in the literature (35). This might result in a more pronounced material loss if the worn material loses adhesion to the bulk material. In contrast, investigations reveal that the platinum-boron-based BMG is more prone to microcracking. Although the mechanism of the platinum-copper-silver-boron-phosphorus and platinum-copper-nickel-phosphorus alloys appears similar, the nickel-containing BMGs Pt42.5Cu27Ni9.5P21 and Pt57.3Cu14.6Ni5.3P22.8 show the smallest wear mark depths. Consequently, a positive effect of nickel on the abrasive resistance can be assumed. Furthermore, the cobalt containing alloy (2 at% cobalt) shows wear mark depths in between the nickel-free and nickel-containing BMGs. Hence, a composition dependant abrasive behaviour could be observed and the nickel containing alloys are suggested to be more suitable for application where abrasion resistance is important. In addition, it was shown that nickel release due to corrosive attack of body fluids is expected to lie far below the limit given by the industrial standard for jewellery applications, ensuring harmless applicability.
The different abrasive behaviour of the two material classes can be explained by extremely localised plastic deformation in BMGs. In the literature, the extent of plastic deformation of metallic glasses depends on the type of measurement. Compression experiments often lead to the formation of multiple shear bands resulting in a distinct plastic regime, for example Pt57.5Cu17.7Ni5.3P22.5, 20% plastic deformation (5). In contrast, the formation of a single shear band usually leads to catastrophic failure in tension experiments (36). Therefore, BMGs are often termed ‘brittle’ although they are highly ductile in a very localised area (shear band). Consequently, the adhesive wear observed for the platinum-phosphorus-based liquids suggest that the pin-on-disc experiments cause very localised plastic deformation leading to peeling off of material which is subsequently plastically deformed and flattened by the pin. This material is continuously transferred to the edges of the wear mark as seen in Figure 5(a) and 5(b). In contrast, Pt49.95Cu16.5Si6.4B24Ge3 shows the formation of microcracks suggesting significantly less ductile behaviour.
The corrosion tests reveal that the element release of the BMG alloys is slightly higher than that of the crystalline benchmark alloys. However, their tarnishing behaviour in artificial saliva is comparable. Pronounced element release is detected for the platinum-boron-based BMG alloy Pt49.95Cu16.5Si6.4B24Ge3 with a high scattering of data.
The oxidative behaviour of the Pt49.95Cu16.5Si6.4B24Ge3 alloy is noticeable with respect to tarnishing. Copper oxide was detected underneath the surface and adjacent to platinum-silicon rich phases after exposure to air at elevated temperatures. Oxidative attack has also been observed on a gold BMG in previous work (37). Growth of silica dendrites into the sample surface and copper oxide dendrites above were detected by annealing experiments in air at the homologous temperature, TH, of about 0.88. The interplay of copper with silicon was concluded to play a crucial role for tarnishing (37). The platinum BMGs show a pattern of internal oxidation of copper compared to the gold BMG. Tarnishing was observed in this work only for the copper and silicon bearing alloy and it can be hypothesised that tarnishing occurs by a mechanism based on the interaction of copper and silicon. The interplay between these elements seems to be different for the platinum BMG compared to the gold BMG alloy. Analogous to the gold-based BMG, the reduction of copper or silicon content might reduce the tarnishing rate (38, 39). However, for a more detailed and in-depth understanding of the underlying mechanism further investigations are necessary.
Due to its worse performance, the Pt49.95Cu16.5Si6.4B24Ge3 alloy was excluded from the investigation of the mechanical and thermoplastic behaviour of the platinum BMG series.
The formability tests show outstanding formability of all platinum alloys due to a fast decrease of viscosity during heating above the glass transition temperature, called kinetically fragile behaviour, combined with a very good thermal stability against crystallisation (30, 40). Compared to other metallic glasses, for example iron- or zirconium-based glasses, the isothermal formability of the examined platinum-based BMGs is two to three orders of magnitude higher. A comparative analysis on thermoplastic formability within the different classes of BMGs is provided in (31). This superior formability together with their high strength predestines them for new innovative filigree and durable design applications in the jewellery sector.
The investigated BMG alloys, except Pt42.5Cu27Ni9.5P21, reach a fineness of 85 wt% platinum, which makes them interesting for jewellery manufacturing. The mechanical analysis of the platinum BMG series demonstrates for all tested alloys a strength and hardness which exceed those of the crystalline benchmarks. These superior properties in the as-cast state enable near net shape casting without the need for subsequent thermomechanical treatment. In the context of filigree geometries, their mechanical properties are very valuable and ensure stability of form.
In comparison to platinum-based benchmark alloys, a more pronounced abrasive damage is expected for the platinum BMG alloys during their lifetime as jewellery items. Furthermore, a composition-dependent tribological behaviour among the platinum BMG alloys is observed, revealing favourable wear resistance associated with the content of nickel. In the context of corrosion behaviour, satisfactory maintenance of appearance during usage, similar to their crystalline counterparts, can be assumed for platinum-based BMG jewellery. The results indicate a higher risk of tarnishing for the phosphorus-free, platinum-boron-based alloy Pt49.95Cu16.5Si6.4B24Ge3 than for the other platinum-based BMGs. Interaction of copper and silicon is assumed to play a crucial role for the tarnishing mechanism.
The results of this work show the suitability of several platinum-based BMG alloys for jewellery application and demonstrate their beneficial properties. In addition to their outstanding elasticity and formability, their thermoplastic formability and high mechanical strength enable new designs, which should further enhance industrial interest and applicability of platinum BMGs.
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This work was financially supported by the German Federal Ministry for Economic Affairs and Climate Action (BMWK) under the IGF programme (project no. AiF-IGF 19979N). The members of the industrial user committee and colleagues in the working groups at LMW and fem are kindly acknowledged for their contribution to this work.
Lisa-Yvonn Schmitt studied Material Science at the University of Saarland, Germany, until 2017 and then started working at fem research institute, Germany, as a project manager. Since summer 2021 she is also a PhD student at the Institute for Applied Materials (IAM-AWP) of Karlsruhe Institute of Technology (KIT), Germany. Her research topics are constitution and thermodynamics of platinum-alloys, bulk metallic glasses and sessile drop measurements of metallic melts.
Nico Neuber is a PhD student at the chair of metallic materials of Saarland University. He obtained a double bachelor’s degree in Materials Science and Mechanical engineering at Saarland University and Oregon State University, USA, and a Master’s degree in Materials Science in Saarbrücken, Germany. His research focuses on the connection of structure with the thermodynamic, kinetic and mechanical properties of metallic glasses and liquids mainly utilising synchrotron X-ray radiation based and calorimetric techniques. Further focus is put on processing technology and casting of amorphous metals.
Miriam Eisenbart graduated in 2009 in Mechanical Engineering from KIT. She then entered the metallurgy department of fem research institute where she worked on gold-based bulk metallic glasses and their mechanical behaviour as well as the development of copper alloys. In 2016 she was awarded her doctorate in engineering. Since 2019 she is the deputy head of the Physical Metallurgy department and has in recent years entered the research fields of additive manufacturing and digitalisation.
Ulrich E. Klotz studied Physical Metallurgy at the University of Stuttgart, Germany, and earned his PhD from The Swiss Federal Institute of Technology (ETH) Zurich, Switzerland. From 1999 to 2007 he worked at the Swiss Federal Laboratories for Materials Testing and Research (Empa), Switzerland. Since 2007 he is Head of the Department of Physical Metallurgy at the fem, Germany. His research interests are thermodynamics and phase diagrams and processing technology including investment casting and additive manufacturing of precious metals alloys.
Ralf Busch studied physics at the Georg-August-Universität Göttingen in Germany. He received his PhD in Göttingen in 1992. In 1993, he went to the California Institute of Technology in Pasadena, USA, as a Feodor Lynen Fellow of the Alexander von Humboldt Foundation. There he started to work in the field of bulk metallic glasses. In 1999, he accepted an Assistant Professorship in Materials Science in the Department of Mechanical Engineering at Oregon State University, USA, and became an Associate Professor there in 2004. He became Full Professor for Metallic Materials at Saarland University, Germany, in 2005.