Igor Velkavrh

Damage Mechanisms of Plasma, Gas and Salt Bath Nitrocarburized Steel in Lab-Scale Sliding Test

Nitrocarburized steel surfaces are often used in tribological forming applications due to their beneficial sliding properties. One typical application field can be found in bending machines where the nitrocarburized layer can withstand massive volume loss and prevent from adhesion of the work sheet material. However, under non-lubricated sliding conditions abrupt failure of the nitrocarburized layer can occur, which results in pronounced increase of friction and wear. In order to characterise the wear mechanisms of nitrocarburized surfaces under non-lubricated sliding conditions, a lab-scale study was carried out. Different nitrocarburizing processes including plasma, gas and salt bath nitrocarburizing were investigated. Oscillating sliding tests with DIN 100Cr6 bearing steel cylinder sliding against nitrocarburized plate were performed at contact pressures typical for bending machines. Evaluation of wear was performed by white-light interferometer with measurements of the wear-scar topography and a subsequent calculation of the average wear depth. A strong influence of the nitrocarburizing process on friction and wear behaviour was observed. This behaviour could be correlated with the microstructure of the compound layer.

Optimisation of Plasma Nitrocarburising for Reducing Wear in Dry Sliding Contacts

In this study, tool steel substrates were plasma-nitrocarburised at different processing parameters and afterwards tribologically tested under non-lubricated sliding conditions. It was observed that the micromechanical properties of the compound layer (thickness, hardness, roughness, surface topography) strongly affect the tribological behaviour of the nitrocarburised surface and can be tailored through the adjustment of the nitrocarburising parameters so that very favourable wear and friction behaviours can be achieved.

Morphology and Tribological Behaviour of Amorphous and Crystalline Aluminum Oxide Layers

Anodization of aluminum (Al) is a well-known process for the production of oxidized Al surfaces. Within the scope of the present work, three different Al-oxide layers, produced with different electrochemical methods, were investigated in view of their layer morphology and tribological properties. For this purpose, a newly developed PCO layer (Innovent e.V.), and two commercially available layers, Ematal and a CERANOD®, were compared. Al-oxide layers produced on flat samples were tested against cylindrical DIN 100Cr6 rings under dry sliding conditions. It was observed that under the selected conditions, the frictional behavior of the Al-oxide layers can be influenced by their structural composition and their surface topography, i.e. is different when their structure is amorphous or crystalline. The newly developed Al-oxide layer, which possesses the lowest surface roughness and a very uniform porosity, provided the longest lasting low friction period. Since γ-Al2O3 was also observed in this layer, it is believed that the structural composition of the Al-oxide layer has additionally influenced the frictional response of these samples.

Tempering-Induced Microstructural Changes in the Weld Heat-Affected Zone of 9 to 12 Pct Cr Steels and Their Influence on Sliding Wear

Increasing amount of tribological applications is working under alternating high/low temperature conditions where the material is subjected to temperature fatigue mechanisms such as creep, softening due to annealing, and at the same time must withstand mechanical wear due to sliding contact with pairing bodies. Steam turbine valves, gate valves, valve heads, stems, seats and bushings, and contacting surfaces of the carrier elements are some examples of such applications. The purpose of the present study is to evaluate the potential of X20 and P91 steels as materials for applications operating under combined effect of mechanical wear and alternating high/low temperature conditions. It was focused on how the microstructural changes occurring in the weld zone affect the wear properties of the selected materials. Generally, with longer tempering time and higher tempering temperature, the number of carbide precipitates decreased, while their relative spacing increased. Before tempering, the morphology of the steel matrix (grain size, microstructure homogeneity) governed the wear resistance of both steels, while after tempering wear response was determined by the combination of the number and the size of carbide particles. After tempering, in X20 steel larger number of stable M23C6 carbides was observed as compared with P91 steel, resulting in lower wear rates. It was observed that for both steels, a similar combination of number density and size distribution of carbide particles provided the highest wear resistance.

Effect of base oil lubrication in comparison with non-lubricated sliding in diamond-like carbon contacts

Abstract Diamond-like carbon (DLC) coatings are, nowadays, used in various mechanical systems, including highly stressed and lubricated applications. However, a lot is still unknown about the interactions between lubricants and DLC coatings. For example, what is the role of base oil in DLC contact? Do base oils adsorb to the DLC surface, or are they only a passive element in the contact? In this study, self-mated DLC–DLC contacts were employed, and steel–steel contacts were used as a reference for the expected lubrication mechanisms, which are well known for steel. In tribological experiments, low velocity boundary lubrication conditions were applied to eliminate any velocity related effects. Low viscosity polyalphaolefin base oil was used as a lubricant, and some tests were made also without the use of the lubricant in the contact. Results showed that the friction of boundary lubricated DLC contacts is always lower than the friction of steel contacts. However, in the low speed boundary lubrication regime, the friction of the lubricated DLC contacts increased in comparison with the non-lubricated conditions, which is the opposite of steel behaviour. Nevertheless, oil was required to stabilise the friction behaviour and to prevent high wear and removal of the DLC coating.

A Viscosity-Based Study of the Tribo-Physical Effects in Boundary-Lubricated DLC Contacts

In about a decade, since interest about boundary lubrication properties of diamond-like carbon (DLC) coatings exists, mainly chemical aspects of the DLC lubrication were investigated, i.e. focusing on interactions between various additives and coatings, while physical aspects of coating-oil interactions were not discussed in a greater extent. To elucidate some of these effects, we have analyzed the friction behaviour of DLC coatings and steel under different boundary lubrication conditions by using polyalphaolefin (PAO) base oils having different viscosity grades. Based on this, we have identified several tribo-physical effects in these contacts and for the purposes of this paper we briefly discuss the physical adsorption between the oils and the DLC surfaces, clearly indicating existence of rather strong physically adsorbed layers, which were able to resist high shear stresses under severe boundary lubrication.Copyright