Comparative Studies on the Cutting Performance of HFCVD Diamond and DLC Coated WC-Co Milling Tools in Dry Machining Al/SiC-MMC

Study on tribological behavior and cutting performance of CVD diamond and DLC films on Co-cemented tungsten carbide substrates

The diamond and diamond-like carbon (DLC) films were deposited on the cobalt cemented tungsten carbide (WC-Co) cutting tools respectively adopting the hot filament chemical vapor deposition (HFCVD) technique and the vacuum arc discharge with a graphite cathode. The scanning electron microscope (SEM), X-ray diffraction spectroscopy (XRD) and Raman spectroscopy were used to characterize the as-deposited diamond and DLC films. To evaluate their cutting performance, comparative turning tests were conducted using the uncoated WC-Co and as-fabricated CVD diamond and DLC coated inserts, with glass fiber reinforced plastics (GFRP) composite materials as the workpiece. The research results exhibited that diamond and DLC coated inserts had great advantages in cutting tests compared to uncoated insert. The flank wear of the CVD diamond coated insert maintained a very low value about 50μm before the cutting tool failure occurred. For the DLC coated insert, its flank wear always maintained a nearly constant value of 70~200μm during whole 45 minutes turning process. The flank wear of CVD diamond coated insert was lower than that of DLC coated insert before diamond films peeling off.

The tribological performance of conventional microcrystalline diamond (MCD) film and diamond-like carbon (DLC) film is investigated comparatively under water lubricating condition. The MCD and DLC film are deposited on cobalt cemented tungsten carbide (WC-Co) substrate using the hot filament chemical vapor deposition (HFCVD) method and the vacuum arc discharge with a graphite cathode respectively. Scanning electron microscopy (SEM), white light interferometer, and Raman spectra are employed to characterize as-deposited MCD and DLC samples. The friction tests are carried out on a ball-on-plate reciprocating friction tester, where the sliding process is conducted under water lubricating condition. Silicon nitride, tungsten carbide, ball-bearing steel and copper are used as counterpart materials. The results indicate that DLC film always exhibits lower friction coefficient than MCD film under water lubricating condition, except the case of sliding against the silicon nitride, in which DLC film is worn out very rapidly and thus leads to the high friction coefficient. The wear resistance of DLC film under water lubricating condition is significantly poorer than that of MCD film. While sliding against silicon nitride, tungsten carbide, ball-bearing steel and copper, its wear rate is calculated as 3.67´10-7mm3N-1m-1, 9.31´10-9mm3N-1m-1, 3.54´10-7mm3N-1m-1, and 4.97´10-8mm3N-1m-1respectively. Comparatively, no measurable wear track can be found on the worn surface of MCD films.

Friction and wear behavior of ion-beam-deposited diamondlikecarbon (DLC) films coated on chemical-vapor-deposited (CVD),fine-grain diamond coatings were examined in ultrahigh vacuum,dry nitrogen, and humid air environments. The DLC films wereproduced by the direct impact of an ion beam (composed of a 3 :17 mixture of Ar and CH4) at ion energies of 1500 and700 eV. Sliding friction experiments were conducted withhemispherical CVD diamond pins sliding on four differentcarbon-base coating systems: DLC films on CVD diamond; DLC filmson silicon; as-deposited, fine-grain CVD diamond; andcarbon-ion-implanted, fine-grain CVD diamond on silicon. Resultsindicate that in ultrahigh vacuum theion-beam-deposited DLC films on fine-grain CVD diamond (similarto the ion-implanted CVD diamond) greatly decrease both thefriction and wear of fine-grain CVD diamond films and providesolid lubrication. In dry nitrogen and in humid air,ion-beam-deposited DLC films on fine-grain CVD diamond films alsohad a lowsteady-state coefficient of friction and a low wear rate. Thesetribological performance benefits, coupled with a wider range ofcoating thicknesses, led to longer endurance life and improvedwear resistance for the DLC deposited on fine-grain CVD diamondin comparison to the ion-implanted diamond films. Thus, DLCdeposited on fine-grain CVD diamond films can be an effectivewear-resistant, lubricating coating regardless of environment.

The absolute photoyields of chemical vapor deposited (CVD) diamond and amorphous hydrogen-free diamondlike carbon (DLC) films, in the range of 140–300 nm, are reported. CVD diamond films exhibit a large photoyield, of a few percent in the range 140–180 nm. DLC films have a 20–50 times lower yield. Post growth hydrogenation is found to substantially increase the photoyield of CVD diamond films. We discuss the applicability of these films as UV photocathodes coupled to electron multipliers based on gaseous charge multiplication.

Owing to their excellent mechanical and tribological properties, like the well-known extreme hardness, low coefficient of friction and high chemical inertness, chemical vapor deposition (CVD) diamond films have found applications as a hard coating for drawing dies. The surface roughness of the diamond films is one of the most important attributes to the drawing dies. In this paper, the effects of different surface roughnesses on the friction properties of diamond films have been experimentally studied. Diamond films were fabricated using hot filament CVD. The WC-Co (Co 6[Formula: see text]wt.%) drawing dies were used as substrates. A gas mixture of acetone and hydrogen gas was used as the feedstock gas. The CVD diamond films were polished using mechanical polishing. Polished diamond films with three different surface roughnesses, as well as the unpolished diamond film, were fabricated in order to study the tribological performance between the CVD diamond films and different metals with oil lubrication. The unpolished and polished CVD diamond films are characterized with scanning electron microscope (SEM), atomic force microscope (AFM), surface profilometer, Raman spectrum and X-ray diffraction (XRD). The friction examinations were carried out by using a ball-on-plate type reciprocating friction tester. Low carbide steel, stainless steel, copper and aluminum materials were used as counterpart balls. Based on this study, the results presented the friction coefficients between the polished CVD films and different metals. The friction tests demonstrate that the smooth surface finish of CVD diamond films is beneficial for reducing their friction coefficients. The diamond films exhibit low friction coefficients when slid against the stainless steel balls and low carbide steel ball, lower than that slid against copper ball and aluminum ball, attributed to the higher ductility of copper and aluminum causing larger amount of wear debris adhering to the sliding interface and higher adhesive strength between the contacting surfaces.

This article presents an overview of the research in chemical vapor deposition (CVD) diamond films on steel substrates. Since the steels are the most commonly used and cost-effective structural materials in modern industry, CVD coating diamond films on steel substrates are extremely important, combining the unique surface properties of diamond with the superior toughness and strength of the core steel substrates, and will open up many new applications in the industry. However, CVD diamond deposition on steel substrates continues to be a persistent problem. We go through the most relevant results of the last two and a half decades, including recent advances in our group. This review discusses the essential reason of the thick catalytic graphite interlayer formed on steel substrates before diamond deposition. The high carbon diffusion in iron would induce severe internal carburization, and then voluminous graphite precipitated from the substrate. In order to hinder the catalytic graphite formation, various methods have been applied for the adherent diamond film deposition, such as pre-imposed various interlayers or multi-interlayers, special controls of the deposition process, the approaches of substrate alloying and so on. We found that adherent diamond films can be directly deposited on Al alloying steel substrates, and then the role of Al alloying element was examined. That is a thin dense amorphous alumina sublayer in situ formed on the alloying substrate, which played a critical role in preventing the formation of graphite phase and consequently enhancing diamond growth and adhesion. The mechanism of Al alloying suggests that the way used to improve hot corrosion resistance is also applicable. Then, some of the hot corrosion resistance methods, such as aluminizing, siliconizing, and so on, which have been used by some researchers examining CVD diamond films on steel substrates, are reviewed. Another way is to prepare diamond-like carbon (DLC) films on steel substrates at low temperature, and then the precipitated graphite from the internal carburization can be effectively avoided. In addition, based on some new findings, the understanding of the diamond nucleation and metastable growth is discussed.

Effects of microstructure of films on CVD diamond X-ray detectors

Diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm−1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm−1 showing the existence of both diamond (sp3 phase) and graphite (sp2 phase) in diamond-like carbon films.

We have investigated the relation of bonding states in as-deposited diamond-like carbon (DLC) films and the incorporation of N and O atoms into DLC films, following plasma treatment of the surface. After deposition with various coating methods and under different conditions, the DLC films were treated with NH3 or O2 plasma. We investigated the changes in bonding states of the films using X-ray photoelectron spectroscopy (XPS). The results suggested that the incorporation due to plasma treatment was affected by the bonding states in the as-deposited DLC films. The C-H/C-C and the sp2 C-C/sp3 C-C ratios in the as-deposited DLC films play an important role in controlling N and O incorporation into the DLC films.

Chemical vapor deposition (CVD) diamond film has long been investigated as an important coating material for a variety of mechanical components, due to its many excellent mechanical and tribological properties, e.g. extremely high hardness, low friction coefficient and excellent wear resistance. The tribological behaviors of CVD diamond films have been known to show significant dependence on both sliding environment and its surface characteristics. The investigation on the effect of sliding conditions like normal load, sliding velocity, counterpart material and lubricant have been reported extensively hi available literatures. Nevertheless, the suidies on the influence of grain size, surface roughness and homogeneity mainly concentrate on the CVD diamond film self-mated tiibosystem [1, 2]. Inadequate attention has been put on the heterogeneous contact of CVD diamond film with metal or ceramics material, which is a considerable frequently used contact type in its mechanical applications, especially for the CVD diamond coated cutting inserts or drills.

Scribing wheel (SW) is an important tool for separating glass panels in thin-film transistor liquid crystal display industry. In this study, unlike the traditional SW completely made of polycrystalline diamond (PCD) or cemented tungsten carbide (c-WC), an alternative partially taking advantage of chemical vapor deposition diamond (CVDD) was newly developed. The fabrication of such unique sandwich-like CVDD-SW combined hot filament chemical vapor deposition (HFCVD), welding, and other machining processes. Both hard CVDD scribing edge and tough c-WC supporting layers contributed to SW structure. CVDD was prepared by adjusting the concentration of methane fed into HFCVD chamber. Morphological observation confirmed the reproducibility of microcrystal diamond (MCD), submicrocrystal diamond (SMCD), and nanocrystal diamond (NCD) diamond. Besides grain size, the existence of columnar structure, the nondiamond carbon content, the residual stress, and I(220)/I(111) ratio of CVDD films were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Based on results, SMCD was predicted as the optimized CVDD for making a scribing edge. After three CVDD films were respectively integrated into SW, this prediction was supported by preliminary scribing test. Selecting Corning-1737 as the cutting object, among three CVDD-SWs and one self-made PCD-SW, only the scribing edge of SMCD-SW kept almost undamaged. The outperformance of our design was thus confirmed.

DLC (Diamond Like Carbon) films show very desirable surface interactions with high hardness, low friction coefficient, and good wear-resistance properties. The friction behavior of hydrogenated DLC film is dependent on tribological environment, especially surrounding temperature. In this work, the tribological behaviors of DLC (Diamond-like carbon) films, prepared by the radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method, were studied in elevated temperatures. The ball-on-disk tests with DLC films on steel specimens were conducted at a sliding speed of 60 rpm, a load of 10 N, and surrounding various temperatures of 25, 40, 55 and 75. The results show considerable dependency of DLC tribological parameters on temperature. The friction coefficient decreased as the surrounding temperature increased. After tests the wear tracks of hydrogenated DLC film were analyzed by optical microscope, scanning electron spectroscopy (SEM) and Raman spectroscopy. The surface roughness and 3-D images of wear track were also obtained by an atomic force microscope (AFM).

Microwave plasma chemical vapour-deposited (CVD) process has been used to grow polycrystalline diamond films over silicon substrates. Diamond-like carbon (DLC) thin films were grown over silicon substrates using a microwave plasma disc reactor. Reactant gases of CH4 and H2 were used in both CVD processes. Some preliminary feasibility tests were performed on the possible applicability of diamond and diamond-like carbon thin films for space-protective applications against artificially simulated electrically actuated plasma drag hypervelocity impact of olivine particles. As-deposited films were analysed by Raman for their chemical nature. The morphology and dimensions of hypervelocity impact craters in diamond and DLC films was also studied by scanning electron microscopy (SEM) and optical microscopy. The velocity of debris particles was determined by high-speed photography using a streak camera. The size of the impact particles was determined by measuring the size of the holes formed in the mylar sheet mounted just above the target diamond and DLC film/silicon and coordinates of the impact sites were determined using the same apparatus.