Figure 3 shows TiN-Gold and DLC-Black coatings on Ti6Al4V substrate, a potential dental implant material. Table 2 lists the basic characteristics evaluated for these coatings. In PVD coatings, residual stress is a critical factor that influences the adhesion of coating at the interface and the thickness to which coating can be grown, as well as the tri-bological, fracture, and fatigue properties.9–12 The standard Daimler-Benz Rockwell indentation test (VDI 3198) offers a quick method for evaluating the residual stress level and adhesion of the coating to the substrate.
In this test, a conical-shaped diamond tip was used to create an indentation on the coated component at a load of 150 kg. After the test, the indentation is evaluated to understand the nature of any coating fracture or delamination.13 According to the above standard, both TiN and DLC coatings in this case have shown HF1 (the best possible) adhesion ratings on Ti6Al4V substrate (see Figure 3) without any delamination across the coating-substrate interface.
Nano hardness testing (at 20 mN; Berkovich tip) and biotribological tests evaluated the structural reliability of the deposited coatings. Biotribological tests were conducted per ASTM F732 and ISO 14242. These test methods standardize the motion, load, and speed profiles relevant to oral biomechanical conditions. Evaluated test results were compared to the properties of similar coatings evaluated in literature.
Table 3 shows the nanomechanical properties of uncoated and coated TiAl6Al4V substrate. Both TiN and DLC coatings deposited in the test system have shown high hardness and elastic recovery. This indicates potential for providing better wear resistance and higher load-bearing capacity. These attributes are essential for mechanical durability at the implant-abutment interface.
How to Get Coatings “Hard Enough” but Not “Too Hard”
Recently, in-situ dental studies have shown that very high hardness is not desirable on implant surfaces as it tends to damage any opposing tooth during occlusal contact. However, the system used in this study is equipped with adequate plasma process control tools to modify the mechanical properties of coatings and provide optimum biomechanical functionality to match internal oral tooth dynamics.
Tribological test performance data for TiN-Gold and DLC-Black is shown in Tables 4 and 5. Data are compared with previously reported findings for similar coatings deposited by other industrial and research sources. For each test, the biological liquids used, as well as the dynamic contact pressure measured, are shown to compare the relative performance of each coating.
The tested PVD TiN coating showed a comparable friction coefficient to the other coatings tested. The amount of wear was also comparable, despite the much higher contact pressure used during the test. The data show that in using this coating system, deposited TiN has similar or slightly improved performance compared with the other coatings. The DLC-black coatings show comparable or lower friction coefficients, as well as significantly decreased wear despite the higher contact pressure during the test.
The coatings evaluated in this study showed improved mechanical and biotribological properties under high stress cyclic loading conditions. They show the required adaptability to address the wear and friction issues associated with dental implants. The TiN coating tested could be a good coating for the bottom of the abutment, where it can provide a smooth surface, fretting wear resistance, biocompatibility, and load-bearing capacity.
DLC may be a good coating on the screw and implant surfaces. Its low friction would be useful during surgical procedures. DLC also has the necessary bone compatibility as well as providing a barrier that prohibits metal ion release at the implant-bone interface. The coating system used in this study can produce thick nanostructured coatings with high deposition rates and low residual stress.
This article was written by Ganesh Kamath, PhD, an R&D scientist with Vapor Technologies, Inc., manufacturer of the VaporTech Cadence thin-film deposition system used in this study. For more information, Click Here.