Jannica Heinrichs

Hydroxylapatite growth on single-crystal rutile substrates.

Titanium is widely used as an implant material. In addition to the bulk properties of titanium, the biological response is to a large degree controlled via the surface. The native amorphous titanium oxide that forms spontaneously on the surface gives a very good biological response. Lately it has been shown that crystalline titanium oxides (rutile and anatase) have in vitro bioactive properties. In addition to its potential for new materials development, this finding also opens up for the possibility of studying the mechanisms of bioactivity on materials with strictly controlled surfaces. In this paper the mechanisms behind the in vitro bioactivity are studied, using rutile single crystals. Three single-crystal rutile substrates: (100), (110), and (001), and a polycrystalline rutile substrate obtained by physical vapour deposition were soaked in a phosphate buffered saline solution for up to 4 weeks. The hydroxylapatite films that formed were analysed by X-ray diffraction, scanning electron microscopy, focused ion beam, and transmission electron microscopy. The hydroxylapatite grew faster on the (001) surface than on the other two. It was also found that on the (001) surface the direction of fast growth in hydroxylapatite was aligned parallel to the surface. This is in contrast to the (110) rutile surface where the fast growth of the hydroxylapatite crystal was directed outwards from the surface. The (100) face had poor adhesion at the interface. The orientations of the precipitated crystallites play a significant role in the faster coverage of the (001) rutile face. Based on the experimental results, a model for the hydroxylapatite growth process is given.

In Vitro Bioactivity of Atomic Layer Deposited Titanium Dioxide on Titanium and Silicon Substrates

A non-bioactive implant device can easily be changed to in vitro bioactive with a thin coating of crystalline TiO2. This crystalline coating can be deposited very thin with great step coverage at a low temperature with Atomic Layer Deposition (ALD). An anatase TiO2 coating was built up atomic layer by atomic layer using TiI4 and H2O as precursors in a hot wall furnace. Several hundreds of cycles resulted in a 10-30nm well defined TiO2 of anatase phase on both Si and Ti substrates. These coatings were shown to be bioactive when immersed in simulated body fluid in vitro, as hydroxyapatite (HA) formed on the surface. The surface roughness of the substrates affected the adhesion of the HA. The adhesion was low on the smooth Si but much better on the 100 times rougher Ti. The ALD technique is promising for coating substrates of all shapes with bioactive crystalline TiO2 at a low temperature.

Initiation of Galling in Metal Forming: Differences Between Aluminium and Austenitic Stainless Steel Studied In Situ in the SEM

High friction and transfer of work material to tool surfaces constitute important industrial problems in forming of many metals and alloys. However, it is very hard to gain a deeper understanding of these phenomena by studying real forming operations. In this paper, we have tried to gain fundamental understanding by avoiding as much as possible of the complexity of real forming. This has been realised by studying the friction and material transfer between well-defined tool material surfaces; uncoated and DLC-coated tool steel, and a needle shaped austenitic stainless steel tip, in situ in the SEM. The tool materials were tested in two conditions; well polished and well polished with local intentional scratches. It was found that work material was immediately transferred to the tool steel surface. When passing an intentional scratch, the local transfer was on a much larger scale, and the friction was higher, but the effect was mostly local. For the polished DLC-coated surface, almost no work material was transferred and the friction was low. An intentional scratch in the polished DLC surface barely influenced the galling behaviour. The present results are discussed in the light of previously published results from an analogous study with aluminium as work material.

Evaluation of TiB2 coatings in cold forming of aluminium

Abstract Aluminium alloy parts are often formed using cold forming. The transfer of aluminium to the tool is a major problem in these operations. TiB2 has low reactivity with aluminium and has shown promising results in other forming tests. Here, cold forming is simulated in equipment comprising a TiB2 coated tool cylinder and an aluminium cylinder in sliding contact. The coated surfaces are prepared to two surface finishes, and the aluminium cylinders were prelubricated or unlubricated respectively. The test is focused on friction level and number of contacts to reach a threshold friction level. The aluminium surface pretreatment was found to be the most important factor; lubrication lowers the friction significantly. The tool surface finish is also important; polishing lowers and stabilises the friction. The TiB2 coating offered 20–30% better galling performance than the uncoated steel. However, this improvement was far from that of the best available diamond-like carbon coatings.

Morphology and composition of tribofilms deposited using tribochemical burnishing technique

Abstract Using a modified burnishing technique, low friction tribofilms can be formed on steel and cast iron surfaces in a controlled manner, before the part is mounted in e.g. an engine. The influence on the underlying substrate as well as the morphology and composition of the tribofilm has been thoroughly investigated in this paper. The coating process was found to smoothen rough plateaus, but not affecting deeper scratches and grooves, e.g. a honing pattern. The influence on the substrate was limited and no significant stresses were introduced in the surface. The tribofilm was mostly amorphous and typically 50–80 nm thick, composed of Fe, O and W, with minor additions of C and S.

Mechanisms of transfer of aluminium to PVD-coated forming tools

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 low-friction 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.

On the Role of Material Transfer in Friction Between Metals: Initial Phenomena and Effects of Roughness and Boundary Lubrication in Sliding Between Aluminium and Tool Steels

In the absence of a lubricant, the friction we measure in sliding contact between metals is typically high and quite erratic, with rapid fluctuations. If we filter out these rapid fluctuations, we can typically also notice slower trends, which can lead to quite dramatic friction changes. Unless careful studies are performed, the cause to this behaviour cannot be understood. How come a material couple cannot be characterised with a specific coefficient of friction? The present paper sets out to add understanding to this area, by conduction and analysing an experimental series involving sliding between a needle-like aluminium tip against tool steel flats. The load is high enough to cause substantial plastic deformation of the aluminium needle; its tip becomes formed by the contact against the tool steel. These small-scale, low sliding distance tests facilitate detailed studies of the initial stages of various friction trends, and the effects of initial surface roughness and shifts of this roughness caused by material transfer between the sliding surfaces. Specifically, the effects on the transfer and friction behaviour from presence or absence of a boundary lubricant film and atmospheric oxygen were studied. It was found that very smooth sliding surfaces can offer low-friction conditions for these metal types. However, the smooth sliding interface is very fragile. In all unlubricated cases tested, it very rapidly (in less than a few mm sliding) became ruined due to transfer, and the friction level correspondingly increased. The boundary lubricant could only offer low friction in cases where the flat steel surface was very smooth. The lubricant also facilitated smoothening of transferred aluminium. As long has been well known, boundary lubrication films typically do not totally hinder direct metallic contact in solid to solid contact. The present results strengthen this view and further suggests that in these direct contacts one of the major friction reducing effects of the lubricant is to efficiently limit transfer, which otherwise acts to make the sliding surface rough.

Transmission Electron Microscopy studies of bio-implant interfaces using Focused Ion Beam microscopy for sample preparation

Biocompatibility of metallic implants is governed by surface characteristics. TEM sample preparation using FIB microscopy very much facilitates our research concerning implant-tissue interfaces as well as studies of bioactive surfaces. This talk will cover some of our research on bioactive titanium surfaces and interfaces of titanium and bone.