Paper Abstract

There is growing international interest in hard biocompatible thin film surface coatings to extend the lifetime of medical implants. Parameters of the physical vapour deposition technique can be utilized to fine tune the microstructure and resulting properties of the growing thin film surface by modifying the adatom mobility of the incoming species. This work investigates the evolution of high hardness and biocompatibility of sputter deposited beta-Ti3Au intermetallic thin film surfaces as a function of growth temperature and pressure. Titanium and gold are sputtered in an optimised 3:1 ratio over glass and Ti6Al4V substrates at varying pressures of 0.3 to 1.2 Pa and temperatures of 25 to 450°C. The microstructure and crystallinity of the deposited films improved with reduction in pressure from 1.2 to 0.3 Pa but development of the β-Ti3Au intermetallic compound occurred at temperatures above 350˚C. The density of the films also increased with reducing pressure, whereas improvement in their columnar structure was observed with increasing substrate temperature. These microstructural changes caused by adatom mobility variation, led to the emergence of superior mechanical surface hardness, reaching a peak value of 12.5 GPa for films grown at 0.3 Pa and 450°C. All thin film surfaces were highly biocompatible with ion leaching levels below 1 ppm, and films deposited at lower pressure exhibited much safer cytotoxic profiles against L929 mouse fibroblasts. This work demonstrates the emergence of high hardness and biocompatibility in Ti3Au thin film surfaces with potential as next generation medical implant coating materials.

How Moorfield products helped:

Thin films of Ti3Au alloy were sputtered onto glass and Ti alloy (Ti6Al4V) substrates using our benchtop NanoPVD-S10A PVD thin film deposition system by magnetron sputtering. Rectangular Ti alloy substrates (76 mm by 26 mm and 1 mm thick) were polished using SiC paper from P240 to P4000 on a wheel polisher to a surface roughness of better than 40 nm, measured in either direction using an Alicona infinity focus surface measurement system. The polished Ti substates were then cut into three coupons measuring 25 mm by 19 mm (1 mm thick) using a guillotine. These Ti coupons were scrubbed, together with a set of glass slides (75 mm by 26 mm and 1 mm thick), using 1:5 Decon 90 agent, followed by ultrasonic bath cleaning with DI water. The cleaned substrates were then rinsed using IPA, acetone and a second ultrasonic bath before being blow dried using a jet of nitrogen. The cleaned substrates were loaded on a substrate holder within the sputtering chamber at a target to substrate distance of 100 mm. The chamber was loaded with 2-inch (50.8 mm) diameter circular targets made from 99.999% pure Ti and Au with the Ti target connected to a DC source, while the Au target was provided with an RF power source to slow down its deposition rate. The chamber was then evacuated to a base pressure better than 5 × 10−4 Pa and the substrate holder was rotated at a constant speed of 5 rpm.

 Figure 1

Fig. 1. Thorntons structural zone model (SZM) showing different zones and variation in thin film texture with variation in deposition pressure and homologous substrate temperature together with a table showing criteria to predict the zone of a thin film in Thornton’s Structural zone model (SZM).

Open Access publication details:

Lukose, C.C. et al. (2023) ‘Adatom controlled emergence of high hardness in biocompatible beta-Ti3Au intermetallic thin film surfaces’, Surfaces and Interfaces, 40, p. 103034. Available at: https://doi.org/10.1016/j.surfin.2023.103034.