Nanomechanical Characterization of Atomic Layer Deposition Coatings for Biomedical Applications

Atomic Layer Deposition (ALD) is a promising technique for the production of\ud biologically safe, wear resistant and corrosion protective coatings for orthopedic\ud applications. In this work, the impact of coating thickness and surface preparation on the\ud hardness (H), elastic modulus (E), wear resistance, and delamination of ALD Al2O3 films\ud is examined.\ud Al2O3 was deposited via ALD at 300 °C using Al(CH3)3 and H2O. 200 nm, 600\ud nm, and 1000 nm thick Al2O3 films were deposited on polished 305 stainless steel\ud substrates. Prior to deposition, stainless steel substrates were cleaned using one of three\ud methods: i) sonication in acetone, isopropyl alcohol and deionized water (AID), ii) AID\ud followed by argon plasma treatment, iii) AID followed by oxygen plasma treatment.\ud Nanowear, nanoscratch, and nanoindentation testing were performed using a Hysitron\ud UBI-1 nanomechanical test system. A Berkovich diamond tip was used for\ud nanoindentation testing to calculate the H and E at the interface of the ALD Al2O3 film\ud with the 305 stainless steel substrate. A conical diamond tip was used to perform scratch\ud testing in order to quantify adhesion through measurement of delamination. The same\ud conical diamond tip was also used for wear testing which characterizes wear resistance of\ud a thin film.\ud Nanoscratch testing indicates excellent adhesion. The 200 nm Al2O3 films do not\ud delaminate even when scratch depth into the sample extends to an average of 175 ± 0.1%\ud of the film thickness. The 600 nm and 1000 nm Al2O3 films delaminate at an average of\ud 102 ± 20% and 42.6 ± 3.8% of their respective film thicknesses. Reactive ion etching\ud (RIE) treated samples also show less scratch resistance than untreated samples that only\ud have the ALD Al2O3 coating. Nanoindentation exhibits a trend of H and E changing from\ud literature ALD Al2O3 values to bulk stainless steel values with increasing penetration\ud depth. Nanowear testing demonstrates that ALD Al2O3 films offer effective protection of\ud the 305 stainless steel substrate, reducing wear by up to 74.1 ± 5% of the resulting values\ud for wear depth of the bulk 305 SS samples. However, no distinguishable difference in\ud mechanical properties or wear resistance was observed due to RIE treatment.\ud From the significant contribution that ALD Al2O3 provides for the improvement\ud of wear resistance and scratch resistance, it may be concluded from this study that ALD\ud Al2O3 exhibits strong potential for the improvement of orthopedic devices