• SHIRSHENDU CHAKRABORTY

Articles written in Bulletin of Materials Science

• Severe wear behaviour of alumina balls sliding against diamond ceramic coatings

At present alumina is themost widely used bio-ceramic material for implants.However, diamond surface offers very good solid lubricant for different machinery, equipment including biomedical implants (hip implants, knee implants, etc.), since the coefficient of friction (COF) of diamond is lower than alumina. In this tribological study, alumina ball was chosen as the counter body material to show better performance of the polycrystalline diamond (PCD) coatings in biomedical load-bearing applications.Wear and friction data were recorded for microwave plasma chemical vapour deposition (MWCVD) grown PCD coatings of four different types, out of which two sampleswere as-deposited coatings, one was chemo-mechanically polished and the other diamond sample was made free standing by wet-chemical etching of the silicon wafer. The coefficient of friction of the MWCVD grown PCD against Al$_2$O$_3$ ball under dry ambient condition was found in the range of 0.29–0.7, but in the presence of simulated body fluid, the COF reduces significantly, in the range of 0.03–0.36. The samples were then characterized by Raman spectroscopy for their quality, by coherence scanning profilometer for surface roughness and by electron microscopy for their microstructural properties. Alumina balls worn out ($14.2 \times 10^{−1}$ mm$^3$) very rapidly with zero wear for diamond ceramic coatings. Since the generation of wear particle is the main problem for load-bearing prosthetic joints, it was concluded that the PCD material can potentially replace existing alumina bio-ceramic for their bettertribological properties.

• Thermodynamics of the oxidation of ZrB$_2$–TiB$_2$, ZrB$_2$–SiC and ZrB$_2$–B$_4$C ceramics

The thermodynamics of the oxidation of three-high temperature ZrB$_2$-based ceramics (ZrB$_2$–TiB$_2$, ZrB$_2$–SiC and ZrB$_2$–B$_4$C) has been studied in order to find the stability domain of zirconium diboride, in terms of temperature, partial pressure of oxygen and composition, in which it is protected against oxidation. In the case of the ZrB$_2$-TiB$_2$ binarysystem, a plot of $\log p$O$_2$ vs. $1/T$ in the temperature range of 500–2000 K and another plot of $p$O$_2$ ($\times$10$^{14}$) vs. $x$TiB$_2$ for $T = 2000$ K are made taking into account the two-extreme possibilities of no solubility and 100% solid solubility between ZrB$_2$ and TiB$_2$, respectively. A plot of $\log p$CO vs. $\log p$O$_2$ is made for 1773 K for the systems ZrB$_2$–SiC and ZrB$_2$–B$_42$C. It was found that the ZrB$_2$–TiB$_2$ ceramics does not have sufficient oxidation resistance in the temperature range of 500–2000 K. ZrB$_2$ of ZrB$_2$–SiC ceramics can be protected under 1 atmosphere oxygen or in air if the liquid borosilicate(with the chosen composition, 70% B$_2$O$_3$–30% SiO$_2$), which is an intermediate product, provides a kinetic barrier to the continuation of oxidation by forming an impervious layer on the exposed surfaces. In contrast, the ZrB$_2$–B$_4$C ceramics does not produce the borosilicate upon oxidation. In view of the volatility of pure liquid B$_2$O$_3$, it is recommended that the ZrB$_2$–B$_4$C ceramics can be used at a lower temperature, perhaps below 1373 K, when the vapour pressure of B$_2$O$_3$ is significantlysmall.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019