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Papers by Staffan Jacobson

Surface & Coatings Technology, Oct 1, 2013

W-S-N films were deposited by reactive magnetron sputtering from WS2 target in Ar/N2 atmosphere. ... more W-S-N films were deposited by reactive magnetron sputtering from WS2 target in Ar/N2 atmosphere. Besides the standard evaluation of composition, structure, morphology, hardness and cohesion/adhesion, the core objective of this paper was to analyze coating tribological behavior. The chemical composition was 34 at.% N, 12 at.% O, 29 at.% W and 25 at.% S and the as-deposited films were completely amorphous. The film thickness was 2.3 µm including the approximately 300 nm thick adhesion improving titanium interlayer. The friction coefficient was lower than 0.003 when sliding in dry nitrogen. The coating showed remarkable wear resistance surviving more than 2 million laps on pin-on-dics. The excellent friction properties were attributed to the formation of a thin tungsten disulfide tribofilm on the top of the wear track of the coating and on the counterpart surface. Moreover, the coating showed ability to replenish damaged areas with solid lubricant. We demonstrated that a

Wiley-VCH Verlag GmbH eBooks, Dec 27, 2007

The Importance of Tool Surface Roughness on the Galling Tendencies in Cold Forming of Aluminium

The Role of Surface Treatment in the Tribofilm Formation on Alumina Friction Drive Components

Influence of hardness and microstructure on the mechanisms of deformation and wear of cemented ca... more Influence of hardness and microstructure on the mechanisms of deformation and wear of cemented carbides for rock drilling

Tribology Letters, Apr 3, 2012

Cold forging is a group of methods effectively used in a number of industrial applications to for... more Cold forging is a group of methods effectively used in a number of industrial applications to form aluminium. Tool life and complexity of the possible shapes are mainly restricted by galling; the uneven tool surfaces caused by lumps of transferred material lead to surface damage on successive parts formed and/or problems in successive forming operations, and high stresses occurring when forming complex shapes. Earlier investigations have shown that improved surface roughness of an uncoated steel tool decreases the amount of adhered aluminium on the tool, but does not decrease the friction in the long run. Testing has also shown that when using conventional forming tools it is very important to have sufficient lubrication to avoid galling. Addition of an optimized lowfriction PVD-coating decreases the galling tendency and increases the tolerance to insufficient lubrication and lubricant film breakthrough, as long as the coating surface is smooth, whilst other similar coatings have little or no effect. The present article further investigates the mechanism behind the transfer of aluminium to the coatings. All coatings were found to be intact after testing, and so the differences were concluded to lie in the chemical properties of the coating and, if these are favourable, then in surface roughness on the micro scale.

Wear, May 1, 2008

Traditional preparation of metallographic cross sections by cutting, polishing and etching has a ... more Traditional preparation of metallographic cross sections by cutting, polishing and etching has a long and successful history of revealing grain sizes, phase composition, gradients, etc. However, the technique is limited with respect to precision in the positioning of the cross section, and it also tends to inadvertently remove brittle constituents and conceal cracks and small details close to ductile phases, that may become smeared out by the polishing. The present paper presents an alternative cross sectioning technique for worn surfaces, developed to automatically reveal weak zones in the surface layer of a material, without the need of precise positioning, and also avoiding inadvertent removal of brittle phases. The technique is applicable to materials that exhibit brittle fracture, and is normally used for characterisation of unworn materials. It is primarily intended for studies in the SEM, since the cross sections produced typically exhibit too coarse topography to be sharply pictured in the light optical microscope. The simple and rapid preparation technique is described and examples from worn cemented carbide rock drills and hot rolls are presented to illustrate some of the capabilities of the technique.

Wear, Sep 1, 1998

The wear volume and wear distribution of chromium steel cam rollers in a high-torque hydraulic mo... more The wear volume and wear distribution of chromium steel cam rollers in a high-torque hydraulic motor has been investigated. The cam roller is a part of a novel silicon nitride/chromium steel journal bearing system. An atomic force microscope (AFM) was used to obtain topographical images of the cam roller surfaces before and after use in a full-scale test of the

International Journal of Refractory Metals & Hard Materials, Dec 1, 2018

Tribology transactions, Oct 28, 2014

ABSTRACT It has been shown many times that cosputtering low-friction coatings of molybdenum disul... more ABSTRACT It has been shown many times that cosputtering low-friction coatings of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) with other elements can improve the structural, mechanical, and tribological properties. To achieve the lowest friction, MoS2 or WS2 should be doped with element(s) improving the hardness and density of the coatings. On the other hand, such elements, or their compounds, should not be present in the outermost molecular layers at the sliding interface. This article suggests that there are important differences between how MoS2 and WS2 coatings respond to or react with doping elements, despite the almost identical structure and behavior of the undoped materials. Two systems have been investigated by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) electron energy loss spectroscopy (EELS), W-S-C-Cr and W-S-C-Ti, and showed significant amounts of oxides, which typically formed a layer just underneath the crystalline WS2 top layer. Further, carbon was almost completely absent in the tribofilms, despite the fact that the as-deposited coatings contained as much as 40–50 at% C. An interesting observation here is that WS2 basal planes surround or embed Fe wear particles, suggesting a relatively strong adhesion or a Fe-S chemical bonding between iron/steel and WS2. The result of this is that the wear particles become pacified and remain in the contact as low-friction material.

Wear, 2021

Abstract Two types of thickener systems, lithium complex (LiX) and polypropylene (PP), were chose... more Abstract Two types of thickener systems, lithium complex (LiX) and polypropylene (PP), were chosen to evaluate graphite, graphene oxide (GO) and reduced graphene oxide (rGO) as grease additives at a concentration of 0.1 wt%. To enhance the additive dispersibility, a mixture of polyalphaolefin oil (PAO) and oil soluble synthetic polyalkylene glycol (OSP-68) was used as base oil. The greases were evaluated in i) silver-coated copper contacts simulating high-load electrical contact applications subjected to fretting and ii) a steel/steel 4-ball wear test equipment. The additives showed no positive effect on friction and wear, neither in fretting nor in 4-ball tests. However, there is a statistically significant difference in wear scar diameter on the 4-ball steel contacts between the two thickener types, LiX and PP. Thus, the PP-grease lubricated steel contacts showed more wear and more tribofilms of iron oxide and grease constituents, indicating more metal-to-metal contact. Hence, the thickener type has a larger impact on the lubricating performance of the grease than do the graphite, GO and rGO additions. The results demonstrate that the addition of graphene-based materials to improve greases is not straightforward. Rather, the grease/graphene-based additive system is complex and many parameters influence the friction and wear results. Hence, more work is needed to obtain a better understanding and possibly better lubricating effects from the graphene-based additives.

Recent developments in thin film synthesis of diamond have facilitated a host of new technical ap... more Recent developments in thin film synthesis of diamond have facilitated a host of new technical applications. These are motivated by the many attractive properties of diamond, for example high hardness, chemical inertness, transparency and heat conductivity. Unfortunately, these properties also make it difficult to fashion complex geometries. Other problems are the severely limited choice of suitable substrate materials and large surface roughness. To reduce these complications, a technology denoted replication and bodybuilding has been developed. The basic principle is to grow the diamond film onto a mold and then build a mechanical support on top of the diamond film. Then the mold is removed. Thereby, a diamond surface with the desired 3D geometry and the same surface roughness as the mold is created.Three potential applications for devices built using the replication and bodybuilding concept have been explored. Grinding tools for hard materials have proved superior to conventional technology in rate of removal as well as in resulting surface finish. Diamond surfaces have also been crafted into ultra-durable dies for injection molding of hard particle reinforced polymers. Initial testing of an abrasive diamond device, intended to make CMP processes more economical and easier to control has successfully been carried out.Diamond and diamond-like carbon is well-known for being “low-friction materials”, but are here demonstrated to actually be “high-friction materials” with the ability to disguise themselves in certain environments, most notably with the aid of water molecules. The mechanisms involved in these variations have been investigated. Using NEXAFS it is shown that high friction sliding is accompanied by changes in the material structure. These changes are induced by surface roughness as well as by strong adhesive forces.Highly hydrogenated carbon coatings, on the other hand, affording super-low friction coefficients (<0.01) under certain circumstances, will suffer an increase in friction in the presence of water.