Polycrystalline Diamond Coating on Orthopaedic Implants: Realization, and Role of Surface Topology and Chemistry in Adsorption of Proteins and Cell Proliferation

Polycrystalline diamond has the potential to improve the osseointegration of orthopaedic implants compared to conventional osteo-implant materials such as titanium. However, despite the excellent biocompatibility and superior mechanical properties, the major challenge of using diamond for implants such as those used for hip arthroplasty is the limitations of microwave plasma chemical vapor deposition (CVD) techniques to synthesize diamond on complex-shaped objects. Here, for the first time we demonstrate diamond growth on titanium acetabular shells using surface wave plasma CVD method. Polycrystalline diamond coatings were synthesized at low temperatures (~400 °C) on three types of acetabular shells with different surface structure and porosity. We achieved diamond growth on highly porous surfaces designed to mimic the structure of the trabecular bone and improve osseointegration. Biocompatibility was investigated on nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) coatings terminated either with hydrogen or oxygen. To understand the role of diamond surface topology and chemistry in attachment and proliferation of mammalian cells we investigated adsorption of extracellular matrix (ECM) proteins, and monitored metabolic activity of fibroblasts, osteoblasts, and bone marrow-derived mesenchymal stem cells (MSCs). The interaction of bovine serum albumin (BSA) and Type I collagen with diamond surface was investigated by confocal fluorescence lifetime imaging microscopy (FLIM). We found that proliferation of MSCs was better on hydrogen terminated UNCD than on oxygen terminated counterpart. These findings corelate to the behaviour of collagen on diamond substrates observed by FLIM. Hydrogen terminated UNCD provides better adhesion and proliferation for MSCs, compared to titanium, while growth of fibroblasts is poorest on hydrogen terminated NCD and osteoblasts behave similarly on all tested surfaces. These results open new opportunities for application of diamond coatings on orthopaedic implants.

Investigation of Friction and Wear Performance of Diamond Coating under Si3N4 Friction Pair

In this study, diamond coatings were deposited through the hot filament chemical vapor deposition method on cemented carbide under different methane concentrations, ranging from 1% to 5%, to analyze the performance of the diamond coatings under different loads and lubrication conditions . Friction and wear tests were carried out using ball-disk friction and wear tester under different loads and lubrication conditions. Scanning electron microscopy, high-resolution Raman spectrometry, optical microscopy, and a surface profiler were used to observe the surface morphology and quality of the coatings after the wear test. The results revealed that the coating prepared under 3% methane concentration was more stable during the friction test than that prepared under other methane concentrations. In addition, the coating prepared under 5% methane concentration had poor adhesion and experienced failure under excessive load. Furthermore, lubricating the friction surface with water effectively reduced the formation of abrasive wear and the friction coefficient, and thus the sample reached the stable stage faster. In addition, the wear rate of the coating under wet condition was approximately 4–5 times less than that under dry friction conditions.

Diamond-Coated Silicon ATR Elements for Process Analytics

Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids or solid samples. However, ATR spectroscopy is rarely found in industrial processes, where inline measurement, continuous operation, and minimal maintenance are important issues. Most materials for mid-infrared (MIR) spectroscopy and specifically for ATR elements do not have either high enough infrared transmission or sufficient mechanical and chemical stability to be exposed to process fluids, abrasive components, and aggressive cleaning agents. Sapphire is the usual choice for infrared wavelengths below 5 µm, and beyond that, only diamond is an established material. The use of diamond coatings on other ATR materials such as silicon will increase the stability of the sensor and will enable the use of larger ATR elements with increased sensitivity at lower cost for wavelengths above 5 µm. Theoretical and experimental investigations of the dependence of ATR absorbances on the incidence angle and thickness of nanocrystalline diamond (NCD) coatings on silicon were performed. By optimizing the coating thickness, a substantial amplification of the ATR absorbance can be achieved compared to an uncoated silicon element. Using a compact FTIR instrument, ATR spectra of water, acetonitrile, and propylene carbonate were measured with planar ATR elements made of coated and uncoated silicon. Compared to sapphire, the long wavelength extreme of the spectral range is extended to approximately 8 μm. With effectively nine ATR reflections, the sensitivity is expected to exceed the performance of typical diamond tip probes.

Cutting performance evaluation of boron-doped and undoped diamond coatings in drilling of CFRP laminates

This study investigated the effect of boron-doped and undoped diamond coatings on the cutting performance of cobalt cemented tungsten carbide (WC-Co) drills when drilling CFRP. Three types of diamond coating, as boron-doped microcrystalline (B-MCD), boron-doped nanocrystalline (B-NCD), and undoped nanocrystalline (NCD), were deposited on specially designed for drilling of CFRP one-shot drills by the hot filament chemical vapor deposition (HFCVD) method. The coating characteristics, such as surface morphology, roughness, carbon structure, and interfacial adhesion, were investigated. Then cutting tests were carried out, and the tool’s flank wear, thrust force, and torque were evaluated. For comparison of cutting performance, non-coated WC-Co drills were used in the tests as well. Furthermore, drilled holes were inspected in terms of peel-up and push-out delamination. According to the results, the B-MCD coated drill presented advantages in tool life, and quality of drilled holes over the NCD and B-NCD coated drills. Also, the results confirmed the adhesion enhanced effect of diamond coating to WC-Co substrate through boron doping of the layer.

Hard-material Adhesion: Which Scales of Roughness Matter?

Background Surface topography strongly modifies adhesion of hard-material contacts, yet roughness of real surfaces typically exists over many length scales, and it is not clear which of these scales has the strongest effect. Objective: This investigation aims to determine which scales of topography have the strongest effect on macroscopic adhesion. Methods Adhesion measurements were performed on technology-relevant diamond coatings of varying roughness using spherical ruby probes that are large enough (0.5-mm-diameter) to sample all length scales of topography. For each material, more than 2000 measurements of pull-off force were performed in order to investigate the magnitude and statistical distribution of adhesion. Using sphere-contact models, the roughness-dependent effective values of work of adhesion were measured, ranging from 0.08 to 7.15 mJ/m2 across the four surfaces. The data was more accurately fit using numerical analysis, where an interaction potential was integrated over the AFM-measured topography of all contacting surfaces. Results These calculations revealed that consideration of nanometer-scale plasticity in the materials was crucial for a good quantitative fit of the measurements, and the presence of such plasticity was confirmed with AFM measurements of the probe after testing. This analysis enabled the extraction of geometry-independent material parameters; the intrinsic work of adhesion between ruby and diamond was determined to be 46.3 mJ/m2. The range of adhesion was 5.6 nm, which is longer than is typically assumed for atomic interactions, but is in agreement with other recent investigations. Finally, the numerical analysis was repeated for the same surfaces but this time with different length-scales of roughness included or filtered out. Conclusions The results demonstrate a critical band of length-scales—between 43 nm and 1.8 µm in lateral size—that has the strongest effect on the total adhesive force for these hard, rough contacts.

Multilayer Diamond Coatings Applied to Micro-End-Milling of Cemented Carbide

Cobalt-cemented carbide micro-end mills were coated with diamond grown by chemical vapor deposition (CVD), with the purpose of micro-machining cemented carbides. The diamond coatings were designed with a multilayer architecture, alternating between sub-microcrystalline and nanocrystalline diamond layers. The structure of the coatings was studied by transmission electron microscopy. High adhesion to the chemically pre-treated WC-7Co tool substrates was observed by Rockwell C indentation, with the diamond coatings withstanding a critical load of 1250 N. The coated tools were tested for micro-end-milling of WC-15Co under air-cooling conditions, being able to cut more than 6500 m over a period of 120 min, after which a flank wear of 47.8 μm was attained. The machining performance and wear behavior of the micro-cutters was studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Crystallographic analysis through cross-sectional selected area electron diffraction patterns, along with characterization in dark-field and HRTEM modes, provided a possible correlation between interfacial stress relaxation and wear properties of the coatings. Overall, this work demonstrates that high adhesion of diamond coatings can be achieved by proper combination of chemical attack and coating architecture. By preventing catastrophic delamination, multilayer CVD diamond coatings are central towards the enhancement of the wear properties and mechanical robustness of carbide tools used for micro-machining of ultra-hard materials.