Patrik Hollman

Friction properties of smooth nanocrystalline diamond coatings

Abstract Smooth nanocrystalline diamond coatings (NDC) have been deposited on spherical cemented carbide substrates. These coatings have been tested in dry and lubricated (water, oil) sliding against cemented carbide (CC), ball-bearing steel (BBS), stainless steel (SS), titanium (Ti) and aluminium (Al). In addition, the same types of coatings were subjected to tests against Ti and Al after polishing NDC. Comparative experiments were performed with self-mated BBS. Dry sliding conditions of as-deposited NDCs against CC, BBS, SS and lightly polished NDCs against Ti and Al gave a friction coefficient of 0.06–0.1. Lubricating the contact generally resulted in a small reduction in friction. The self-mated BBS showed a friction of 0.8, 0.4 and 0.14 when tested in dry-, water- and oil-lubricated conditions, respectively. A very low wear was detected for the NDC, and the counter surfaces displayed practically no wear at all. In dry sliding against SS, the wear resistance of the diamond was roughly 100 times higher than that of self-mated BBS tested with oil lubrication. All three test conditions of as-deposited NDCs against Ti and Al gave a high friction coefficient (0.3–0.6) and seizure except for oil-lubricated Al where the friction was 0.11.

Direct current bias applied to hot flame diamond deposition produces smooth low friction coatings

Abstract As-deposited diamond coatings generally have a high surface roughness which results in a high friction coefficient and extensive wear of the counter material in sliding contact. Therefore several methods for smoothening diamond coatings have been proposed, such as laser polishing, molten metal etching, thermochemical polishing and mechanical polishing. All these methods have some disadvantage e.g. long processing time or high processing temperature. Furthermore, they are all post-deposition treatments i.e. the manufacture of these coatings requires at least two processing steps, deposition and smoothening. With the present method which combines d.c. bias with hot flame diamond deposition, a smooth diamond surface is produced during the actual growth of the film. No post-deposition treatment is necessary. The surface roughness is not dependent on the coating thickness which means that thick coatings with smooth surface can be produced. In fact, the method has a smoothening effect, i.e. rough surfaces can be made smooth. The method is comparable to conventional hot flame deposition of diamond as to growth rate and cost of producing the coatings. The coatings have a nano-crystalline structure and a surface roughness of Ra = 25 nm, and result in a friction coefficient of 0.1 or less in dry sliding and about 0.05 in water-lubricated sliding against cemented carbide. Their wear resistance is virtually the same as that of conventional diamond films.

Deposition and microstructure of PVD TiNNbN multilayered coatings by combined reactive electron beam evaporation and DC sputtering

Abstract Well-adherent TiN/NbN multilayer coatings were deposited on high speed steel and cemented carbide substrates in a high vacuum coating equipment using a reactive hybrid deposition process consisting of a combination of electron beam evaporation (Ti) and D.C. magnetron sputtering (Nb). Homogeneous TiN and NbN layers as well as four different multilayer TiN/NbN coatings with periodicity 500/500 nm (TiN/NbN), 100/10, 10/100 and 10/5 nm were deposited. Analytical techniques including X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy and Auger electron spectroscopy were used to characterise the phase composition, microstructure and the chemical composition of the coatings. In addition the microhardness was determined. The phase of the homogeneous TiN and NbN coatings as well as for individual TiN and NbN layers in the multilayer coatings was predominantly the cubic NaCl-structure. In addition the 10/5 coating was found to have a superlattice structure. All coatings showed a dense, columnar microstructure and were slightly overstoichiometric. The highest hardness (≈ 3400 HV) was found for the 10/5, 10/100 and the single layered NbN coating.

Diamond replicas from microstructured silicon masters

Abstract We are developing a microstructure technology for thick film diamond replicas, using deposition by hot filament chemical vapour deposition (CVD) on microstructured silicon. This technology is primarily intended to make micromechanical structures for microstructured carriers, fluidic cooling systems, systems for biochemical analyses and processes, and moulds for thermoplastic and metal microstructures. With thick film deposition ridges, trenches, and capillary channels with high resolution coverage and low roughness, rms

Residual stress, Young's modulus and fracture stress of hot flame deposited diamond

Abstract Chemical vapour deposition (CVD) diamond coatings deposited on various substrates usually contain residual stresses. Since the residual stress affects the adhesion of the coating to the substrate, as well as the performance of the coating/substrate composite in many technical applications it is of importance to study the magnitude of these stresses. In the present study the hot flame method was used to deposit diamond coatings on cemented carbide inserts by scanning the surface with a nine flame nozzle. By varying the oxygen to acetylene flow ratio and the deposition time coatings of different qualities and thicknesses were obtained. The residual strain/stress of the coatings was measured by three different methods: X-ray diffraction using the sin 2 (Ψ) method, Raman spectroscopy and disc deflection measurements. To extract the residual stress from the strain data the Youngs modulus was obtained from bending tests of diamond cantilever beams manufactured from free standing diamond films. The latter technique was also used to determine the fracture stress of the diamond films. All deposited coatings displayed a residual compressive strain/stress state. The residual strain in the diamond coatings did not vary with coating thickness (1.5 μm to 20 μm) but was found to increase from −1.8 × 10 −3 to −2.2 × 10 −3 with decreasing diamond quality. The compressive residual stress was found to decrease from −2 GPa to −1.3 GPa with decreasing diamond quality. This is mainly due to a decrease in Youngs modulus (from 1.1 TPa to 0.6 TPa) with decreasing diamond quality. Also the fracture stress was found to decrease (from 1.8 GPa to 0.8 GPa) with decreasing diamond quality. The three methods used for measuring the stress state in the coatings, X-ray diffraction, Raman spectroscopy and deflection measurement, all give the same result. The deflection technique has the advantage that no information about the elastic properties of the coating is needed, whereas Raman spectroscopy has the best lateral resolution (≈5 μm) and is the fastest method (≈5 min).

Diamond coatings applied to mechanical face seals

Abstract The desired properties of a mechanical face seal are low friction and low wear rate. To fulfil these requirements the seal material needs a low friction coefficient, high hardness, high corrosion resistance, high thermal conductivity and a high thermal shock resistance. Diamond displays a low friction against almost all materials. Furthermore, diamond shows the highest hardness of all materials, is extremely corrosion resistant, has the highest thermal conductivity (four times that of copper) and has a high thermal shock resistance. Therefore, diamond has the potential of becoming a very competitive seal material. The use of diamond in this kind of application has not yet been realised since it is an expensive material and very difficult to shape into complex geometries. However, it has recently become possible to deposit diamond coatings onto various substrate materials with the desired shape and thereby give the component diamond properties. The objective of this study is to investigate the performance of diamond coated cemented carbide as a face seal material. Laboratory dry sliding tests have been performed as well as component tests with water as the pressure medium. Commercial face seals were used as reference. The results show that diamond coatings considerably lower the friction in mechanical face seals and are therefore an interesting alternative to seal materials commercially available today. The benefits from using diamond coatings include lower running cost, increased reliability and the possibility to use a more simple seal design.

Tensile testing as a method for determining the Young's modulus of thin hard coatings

Abstract The present paper describes and evaluates a tensile testing method for determination of the Youngs modulus of thin hard coatings. When applied to PVD TiN and NbN coatings on stainless steel foils, the elastic modulus obtained through force—strain plots ranged between 380 and 425 GPa for TiN coatings, and was 350 GPa for the single NbN coating tested. It is concluded that the method is easy to perform and allows reliable determination of the Youngs modulus of thin hard coatings, and that it can be applied to almost any kind of coating on a wide range of substrate materials.

CVD-DIAMOND COATINGS IN SLIDING CONTACT WITH AL, AL-17SI AND STEEL

Abstract Polycrystalline diamond coatings were deposited on cemented carbide drill bits with the hot flame method. The coated bits were then subjected to sliding tests in a pin on cylinder test equipment. Carbon steel, Al-17Si and Al were used as counter materials. Diamond coatings with a low surface roughness have a low friction coefficient and cause only a small transfer of counter material compared to cemented carbide, especially against the aluminium alloy. Diamond coatings also have an extremely low wear rate against Al-17Si compared to cemented carbide which makes diamond coated tools suitable for machining Al-alloys. Diamond coatings display a high wear rate when sliding against steel due to a phase transformation to amorphous carbon.

Tribological evaluation of thermally activated CVD diamond-like carbon (DLC) coatings

Abstract Diamond-like carbon coatings have been the subject of expanding technological interest for wear resistance and low friction applications during recent years. In this study, diamond-like carbon coatings were deposited by thermally activated chemical vapour deposition. The deposition temperature was 900°C and CH 2 I 2 was used as a precursor. Tribological evaluation was performed by surface roughness measurement together with hardness, micro abrasion and friction. Furthermore, the residual stress of the coating was evaluated by a deflection method. The coating was analysed with nuclear reaction analysis, scanning electron microscopy and atomic force microscopy. The coating was very smooth ( R a = 1 nm) and free from pores. A high abrasive wear resistance slightly worse than physical-vapour-deposited TiN was measured. It exhibit a hardness of 2800 HV and a friction coefficient of about 0.2 in air (30–50% relative humididty). Furthermore, the coating has a compressive residual stress of 220 MPa. Hydrogen content and density of the coating were 9 at.% and 1.8 g cm −3 , respectively.

Solid particle erosion of hot flame-deposited diamond coatings on cemented carbide

Abstract Diamond coatings were deposited on cemented carbide substrates using the hot flame technique developed by Hirose and Kondoh (Y. Hirose and N. Kondo, Proc. Japan Appl. Phys. Spring Meet., 1988, p. 434). The coatings were subjected to erosion testing with alumina particles at velocities of 46 and 72 m s −1 . Cemented carbide, TiN/TiC- and Al 2 O 3 /TiC-coated cemented carbide were used as reference. The most important parameter controlling the erosion resistance is the diamond coating thickness. With a sufficient coating thickness (6 μm) the diamond-coated samples outperformed the reference materials whereas a too thin coating (1.5 μm) yielded a lower erosion resistance. The erosion process is initiated by the formation of a hole in the coating. Thereafter the erosion proceeds by a spalling mechanism. It is proposed that the initial holes are formed during deposition or by fragmentation due to point defects in the coating and that the driving force for the spalling is the relaxation of residual stresses in the coating by crack propagation in the coating and in the coating/substrate interface.