AVS2015 Session TR-ThP: Tribology Poster Session

Hard thin films such as titanium nitride and titanium aluminum nitride films are widely used to improve the durability of cutting tools, punches, and dies that are extensively used in the manufacturing industries. In general, the hardness of hard coating films is about three to five times that of tool steel surfaces causing a remarkably high stress that would eventually lead to the detachment of the hard coating. One way of improving the adhesion of hard coatings is to increase the surface hardness of the tool steel, narrowing the gap, to maintain a reasonably stronger bondage.

In this research, neutral nitriding operation was performed on the SKH 51 tool steel surface before TiAlN coating. Neutral nitriding is a plasma process that allows only the neutral species to diffuse into the interstitial layer of the tool steel surface. The process delimits the formation of the undesirable brittle compound layer to a much higher treatment time while keeping the as-finished surface conditions. The scratch test was performed to provide a measure of coating/tool steel adhesion.

The results of our experiments show that in nitriding SKH 51 tool steel for 6 hours, the hardness of the tool steel surface was increased by about two times, 1400 Hv compared to the untreated sample (750 Hv). The scratch tests show that a well-defined failure of the coating occur at a higher critical load in tool steel samples that are neutral nitrided compared to samples without the pre-treatment. Our results demonstrate that neutral species based nitriding is effective for high adhesion of hard coating materials on tool steel surfaces.

Stainless steels have very good corrosion resistance but the lower surface hardness poses marked limitation on the range of tribological applications that can be envisaged. Here plasma nitriding treatment was performed to improve the hardness without altering the corrosion resistance of austenitic AISI 304 stainless steels.

In nitriding the AISI 304 stainless steel, the passive Cr₂O₃ layer that protects the surface from corrosion is removed first as it hinders the diffusion of nitrogen species into the bulk of the stainless steel material. The removal of the passive layer was done by argon sputtering and the subsequent nitriding was performed in electron beam excited plasma apparatus under nitrogen and hydrogen atmosphere. The sample temperature was controlled at 400, 450, and 500 degree centigrade. Treated and untreated samples were characterized by means of morphological analysis, Vickers hardness measurements, optical microscope, and x-ray diffractometry.

The results of our experiments show that in neutral nitriding of stainless steel under low temperature, 400 degrees centigrade, the surface hardness increased to 780 Hv, more than two times, that of the untreated surface. Moreover, the cross sectional images show clear nitrogen expanded austenite layer without the formation of compound layer. Measurements of surface roughness and conditions of the surfaces were reasonably maintained. The above results indicate that stainless steels can be used as sliding or meshing mechanical parts in environments such as vacuum chambers and underwater machinery.

This study clarifies the formation mechanisms of thin layer on the counter material when nanocompsite coatings consisted of nanoparticles and diamond-like carbon were sliding against steel ball in air. When low friction was obtained in such combination, thin metallic film was formed on the counter material. What we could find is that such metallic film does not contain oxygen less than the one coming from native oxidized surface.

Hybrid deposition system composed of CVD and PVD sources allows us to make nanocomposite structure based on diamond-like carbon (DLC) coatings. Thanks to magnetron sputtering system, target material in sputtering source is extracted as a form of nanoparticle, and its size varies depending on deposition condition and type of material. During sputtering process, hydrocarbon plasma is generated, and hydrogenated amorphous carbon phase is formed. Typical structure of the composite coatings is that nanometer-sized metallic particles are well dispersed in amorphous carbon host matrix. Friction tests are conducted under vacuum or dry conditions. Various analytical techniques including transmission electron microscope (TEM), secondary ion mass spectrometer (SIMS), Raman spectroscopy, Scanning Electron Microscope equipped with energy dispersive X-ray spectrometer (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).

Once iridium was selected as an inclusions in diamond-like carbon host matrix, Ir-containing DLC showed friction coefficient of less than 0.1 despite friction coefficients of Ir and DLC showed ~ 0.3. In-situ electrical contact resistance measurements revealed that such coating provides not only low friction coefficient but also low electrical contact resistance, several tens ohms. When low friction and electrical contact resistance was achieved, metallic thin film was formed on the counter material. EDS analyses revealed that chief material forming the metallic film is Ir. It could be understood that nanoparticles formed metallic thin film on the counter materials during friction. It is worth noting that such transfer film does not contain oxygen even friction tests were conducted under air. Same phenomena could be observed with Cu- and Ag-containing DLCs. Even for both cases, metallic transfer films were formed during friction tests, and we could not detect oxygen from the transfer by EDS. Possibly, tribochemical reaction occurred and oxygen from air reacts to carbon in DLC, then gas phase or wear particles of carbon-oxides were formed. Details on a possible chemical reaction between the coatings and air will be discussed at the conference.

Solid lubricants are usually used at severe environment (ex. vacuum). Among these lubricants, MoS₂ is widely used for many mechanical systems and shows low friction by forming transferred layer with MoS₂ lamellar structure. However, one of the drawbacks of MoS₂ is that it must be kept in dry nitrogen condition to avoid oxidation of molybdenum-disulfide.

We have proposed amorphous carbon coatings containing Molybdenum disulfide (MoS₂-DLC) as a new solid lubricant. It is well known that amorphous carbon coating is used for a gas barrier coating decreasing gas permeability. So, we have made the coatings with nanocomposite structure that MoS₂ clusters with several nm in diameter are embedded in hydrogenated amorphous carbon (a-C:H) host matrix. It was reported that friction coefficient obtained by MoS₂ sliding against SiC ball decreased with increasing MoS₂ concentration It is worth noting that MoS₂-DLC with proper MoS₂ concentration showed lower friction than MoS₂ coatings. Further reduction of friction could be obtained by changing a deposition parameter leading to a modification of host matrix of amorphous carbon. Therefore, not only concentration of MoS₂ but also properties of amorphous carbon host matrix are very important for achieving low friction.

Combinatorial technology has much attention as the effective method for the development of novel functional materials. Also, the combinatorial technique have been used as one of the analysis methods in tribology research.

In this study, we were performed crystal structure analysis by combinatorial optimization of crystal orientation for tribology (COCOT) technique at the sliding surface of after the friction measurement (while changing the load continuously) of metal oxide thin film such as ZnO and Cr₂O₃.

As a result, it is a correlation in the crystal optimization of the sliding surface and the counter ball materials had an effect on the friction coefficient of the thin films.