Wear & Tear

Sponsored by:

By Teresa Fryé, Ulrich E. Klotz, & Tiziana Heiss

(Originally published July 2021)

Editor’s Note: The following excerpt is from “Wear Resistance of Platinum and Gold Alloys: A Comparative Study,” which appears in the July 2021 issue of Johnson Matthey Technology Review (click here read the full study).

Within the industry, it’s long been said that platinum jewelry items tend to outlast their gold counterparts, at least when subjected to human wear. Whether it’s obvious erosion of prong tips or the gradual thinning out of wedding bands to the point of fracture, gold alloys are acknowledged by many in the industry as losing mass at a faster rate than platinum alloys.

Given the high costs of precious metals and the intrinsic value of the jewelry made from them, durability is a top concern in our industry. From the physical costs of replacement to simply being irreplaceable in the mind of a consumer attaching sentimental value to an item, being attentive to longevity of the precious metals we work with benefits both people and planet.

Generally, the wear and hardness of pure metals are reciprocal: wear decreases as hardness increases. But that’s not always the case with alloys. For example, a 1983 study by Degussa AG in Germany on the abrasive wear of gold jewelry alloys found no correlation between hardness and wear resistance. In fact, it was demonstrated that sometimes the softer alloys showed higher wear resistance. The study’s authors concluded that properties such as ductility, toughness, and brittleness can strongly influence the wear resistance of an alloy. But the literature remains sparse, and a deeper understanding of the underlying mechanisms behind anecdotal reports of platinum’s superior wear resistance remains elusive. That’s why we set out to study wear resistance of platinum and gold alloys. On the following pages, we share how we put platinum and gold alloys to the test in order to see whether results would align with the anecdotal.

There are countless chemicals and mechanical forces that jewelry is subjected to during normal human wear. Therefore, any standardized test that attempts to replicate such conditions can be seen only as an approximation of what actually happens in real-life conditions.

Most of the world’s jewelry is cast, so cast samples were the best way to capture the broadest representation of the consumer’s experience. The internal features and qualities of cast pieces, (e.g. porosity levels and grain size) are inherently less homogeneous than say, rolled sheet or drawn wire. With the latter, one can influence internal microstructures, and therefore improve mechanical properties such as strength, hardness, and ductility. By using cast pieces in our testing, we did not seek to test the most optimal condition of an alloy, but rather the most common and arguably the least optimal.

We tested six alloys: one 95 percent platinum and 5 percent iridium, one 95 percent platinum and 5 percent ruthenium, two nickel white golds (14k and 18k), and two palladium white golds (14k and 18k). A total of one coupon (Figure 1) and five cubes (Figure 2) were produced in each alloy, and all sample surfaces were polished according to standard industry practices. The samples were then subjected to a series of scratch, chemical corrosion, and wear tests.

Scratch Test
In order to identify possible wear mechanisms for the alloys, we sought to better understand the nature and role of the individual scratch. There are two distinct types of scratches in metals: micro-cutting and micro-ploughing. Micro-cutting is the removal of material by hard particles, where the volume of the detached material equals the volume of the scratches. In contrast, micro-ploughing is the result of plastic deformation forming bulged areas of material along the scratches, where much of this material is retained rather than shed. Put simply, micro-cutting removes material, while micro-ploughing moves material.

The test was done by producing coupons in each alloy that could be scratched under controlled loads. The samples were first ground plane-parallel on both sides and then polished on the side designated for testing, followed by scratching under both constant and increasing loads.

A piece of adhesive tape was then applied directly and uniformly to each scratch in order to embed and remove any spalled material. This allowed us to compare the susceptibility of the platinum and gold alloys to scoring damage. The tapes were then examined with energy dispersive x-ray spectroscopy to confirm the composition of the metal chips as well as to characterize the amount of chipping.

Through scanning electron microscopy analysis (SEM), we saw evidence that the depth of the scratch was impacted by the hardness of the alloy. As expected, the softer the alloy, the deeper the scratch and the more material that was displaced. In the case of the soft platinum iridium, the displaced material was concentrated at the edges and the tip of the scratch, which is typical for micro-ploughing.

In comparison, the gold alloys showed much less displacement of material toward the scratch tip, but also significant chipping along the scratch. This indicated that the deformation mechanism is still dominated by micro-ploughing but also shows a tendency toward micro-cutting. This was especially true for 14k nickel white gold (Figure 3).

High-density particles were detected on all the tape lifts, though the amount varied significantly by alloy. The platinum alloys as well as both palladium white gold alloys exhibited very few particles on the tape lifts, whereas both nickel white gold alloys exhibited a considerably higher number. These chips are believed to cause the actual mass loss during scratching (Figure 4).

Corrosion Test
The corrosion test involved applying artificial human sweat to the test cubes followed by heating them in a closed chamber at 40°C for 24 hours. We wanted to consider the role of corrosion as a contributor to wear since gold alloys typically contain significant amounts of oxidation-prone base metal elements. Since the platinum alloys were made up of pure and noble platinum group metals, we didn’t expect the corrosion tests to have any effect but opted to include them in the tests for the sake of completeness (Figure 5).

We assessed corrosion by optical microscopy following each test. As expected, both platinum alloys showed no visible change following corrosion testing. Similar results were seen in the 18k nickel white gold and in both the 14k and 18k palladium white golds, none of which exhibited visible corrosion after any of the five test cycles. It’s likely that these three white gold alloys have higher corrosion resistance due to their greater noble metal content. However, given the limited time of the testing combined with its static nature, we can’t rule out any potential effects of longer-term corrosion impacts given their non-noble content, especially under more complex human wear conditions.

The one alloy that demonstrated clear visible corrosion was the 14k nickel white gold (Figure 6), which contains high amounts of nickel, copper, and zinc. The initial wear test exposed porosity on the surface, which likely contributed to a concentration of corrodents to speed up the corrosion process. If that’s the case, casting quality level may also be a contributor to reduced wear resistance, especially in alloys with low corrosion resistance.

Wear Test
The goal of these tests was to determine both mass loss and volume loss through a combination of abrasion and polish testing. We devised two separate tests that we felt would best represent the more likely jewelry experiences during human wear. The abrasion test used sand and stone media while the polish test employed nutshell media. Both media were calibrated and laboratory grade. The samples were placed in motorized rotating drums, and to prevent the samples from contacting each other during testing, they were anchored to both ends of the drum frame with a nylon cord (Figure 7).

Five cycles of testing were done sequentially, starting with abrasion, followed by corrosion, and completed with polishing. Two additional cycles were then done but involved only the polishing test. The cube samples were weighed and characterized by optical microscopy and Vickers microhardness testing before and after each test. The samples were cleaned in an ultrasonic bath with ethanol to ensure any media clinging to the surface was removed. SEM was also used to characterize the surface appearance of the samples.

During the abrasion test portion no remarkable difference between the alloys was observed and overall mass loss was extremely small. The 14k palladium white gold did show slightly higher wear compared to the other alloys, but the mass loss was only 0.00216 grams, or 0.8 percent of original mass. The low mass loss is due to the very short abrasion time (Figure 8).

The more interesting measurement for wear testing of materials with widely varying densities is volume loss, which better demonstrates any visual impact of mass loss. The platinum alloys have a density of ca. 20g/cm³; 18k gold alloys are at ca. 15g/cm³; and 14k gold alloys are at ca. 13-14g/cm³. While mass loss is very similar for all alloys, the volume loss differs more due to these distinctly different density levels.

We calculated volume loss by dividing mass loss by density and grouped the alloys by metal and karatage. The platinum alloys showed the lowest volume loss, followed by the 18k alloys, and then the 14k alloys. The total volume loss in the abrasion portion of the testing was very low, with a maximum value at only 0.0005 mm3 (0.03 percent of the original volume).

For the polishing test with nutshell media, the mass/volume loss per unit of time was comparable to the loss rate in the abrasion test. Mass loss was demonstrated to increase linearly with increasing polishing time (Figure 9). The platinum alloys again show the lowest mass loss with total loss after 244 hours of combined testing at less than half that of the 18k palladium white gold, which showed the highest mass loss in the group. The mass loss of 14k and 18k nickel white gold alloys lies in between the two palladium white gold alloys. Total mass loss was 0.013 g for the platinum ruthenium (lowest value) and 0.031 g for the 18k palladium white gold (highest value). These are still very small amounts equal to 0.03 percent for the platinum ruthenium and 1.1 percent for the 18k palladium white gold. However, when we consider volume losses these differences take on much greater significance. The volume loss of both platinum alloys is a factor of 3x lower compared to 18k palladium white gold, and a factor of about 2x lower compared to 14k palladium white gold and both 14k and 18k nickel white gold alloys.

Surface Quality
To assess surface quality of the alloys, we used stereomicroscopy to examine the corners and edges of the test samples. All the samples were examined following the first two cycles and again after the seventh and final cycle. Figure 10 shows the samples that had the highest and lowest volume loss—platinum ruthenium and 18k palladium white gold.

After the first two cycles, the platinum ruthenium samples still have a very well-defined cube shape; after seven cycles, there is only a very slight rounding of the corners.

Figure 10: Comparison of surface conditions of platinum ruthenium (top row) and 18k palladium white gold (bottom row)

The 18k palladium white gold samples began the process with a less-defined cube shape due to hand polishing prior to testing. In addition, the surfaces of the samples were somewhat uneven. After the first two cycles, edges and corners appear more rounded, and the condition became more pronounced after the seventh cycle. Clearly, mass loss had occurred during testing.

Mechanical Properties
Tensile testing was performed to determine whether strength and ductility measures might play a role in mass loss. As seen in Table 2, we did not find any significant correlation between tensile properties or hardness and mass loss. While it appears that high hardness is not an indicator for low mass or volume loss, the opposite can’t be concluded either. Rather, the situation appears to be more complex and depends upon the mechanism of mass loss during wear testing.

The alloys exhibited very different hardness levels, with the samples of the 14k palladium white gold showing a spread of more than 10 percent, indicating an inhomogeneous microstructure, possibly due to porosity. It can be assumed that the microstructure of an alloy plays an important role on wear behavior. Micropores along scratches will act as points of stress concentration and may cause the chips to break free. Increased levels of microporosity are likely to favor micro-chipping over micro-ploughing, suggesting increased mass loss due to metal chips. However, additional testing would be needed to prove this hypothesis.

Our wear testing revealed significant differences in mass and volume loss between platinum and white gold alloys that aligns well with anecdotal evidence widely embraced by the jewelry industry. The volume loss of both platinum alloys is a factor of 3x lower compared to 18k palladium white gold, and a factor of about 2x lower compared to 14k palladium white gold and both nickel white gold alloys. And while our testing did not allow for a thorough understanding of the precise mechanisms behind the different wear behaviors, the role of the individual scratch (micro-cutting versus micro-ploughing) offers strong hints.

DeGussa was clearly on to something in its 1983 study, and this data adds vital support to its claims. And for those of us obsessed with atoms, microstructures, and mechanical properties, the work has only just begun.