Richard Westergård

Wear of surface treated dies for aluminium extrusion : a case study

Abstract Dies for extrusion of aluminium alloys are exposed to severe thermal, chemical and mechanical conditions. Extrusion dies are usually made from hot work tool steels such as AISI H13. In order to reduce wear they are almost always surface treated by various forms of nitriding. Surface-coating by physical vapour deposition (PVD) or chemical vapour deposition (CVD) is currently being introduced as a means to further improve the wear resistance. The present study aims at elucidating the life limiting wear mechanisms of gas nitrided tools, and duplex coated tools. The latter has been plasma nitrided and subsequently PVD coated with CrN in the same process. The duplex coating is included for its potential to effectively reduce the wear of extrusion dies. It is estimated that duplex coating prolongs the tool life at least five times as compared to conventional nitriding. The reason is that the superficial PVD-layer shows a combination of high resistance to mechanical wear and resistance to corrosive attack by hot aluminium. Eventually, the PVD-layer fractures and delaminates due to fatigue. When the nitrided substrate is exposed, the wear accelerates significantly.

An evaluation of plasma sprayed ceramic coatings by erosion, abrasion and bend testing

Abstract The mechanical properties of a series of plasma sprayed ceramic coatings made from alumina, chromia and alumina–titania precursor powders have been evaluated in particle erosion, two-body abrasion and bend testing. For the alumina coatings, both monocrystalline (sapphire) and polycrystalline alumina powders of varied grit sizes were used. Also, coatings of chromia and a mixture of alumina and titania were evaluated. The coatings were deposited with an axial injection spraying technique. Clear differences were found between the investigated coatings. Among the alumina coatings, those deposited from sapphire powders gave the best results in the wear tests. An addition of titania to alumina had a favourable effect in most tests. In abrasion, one chromia coating displayed a wear resistance comparable to the dense sintered alumina reference. The erosion rates were, however, several times higher for all sprayed coatings compared to the reference alumina.

Cohesion in plasma-sprayed coatings - A comparison between evaluation methods

Abstract The integrity, and especially, the cohesive strength of alumina plasma-sprayed coatings prepared from a range of precursor powders of different size and crystallinity has been investigated by several different methods. Several chromia and alumina-titania coatings, as well as a sintered bulk alumina, have also been studied for comparison and reference purposes. Methods of characterisation have included assessment of the coatings resistance to open tribo-system wear by dry particle erosion and abrasion on the one hand, and their resistance to mechanical failure during controlled scratch testing and tensile fracture in four-point bending, on the other. Controlled scratching was carried out on both top- and cross-section, with the cross-sectional scratching done inside a scanning electron microscopy (SEM). Bending was carried out, in ambient conditions, using test equipment small enough to be placed in the SEM, in order to observe the progressive cracking behavior and failure of the coatings. Some correspondence was found between most of the techniques/methods studied, but erosion and abrasion with large, hard particles and scratch testing on both top surfaces and cross-sections provided the best correlation. It is concluded that these are the most promising methods found so far to rank the cohesion characteristics of lasma sprayed coatings.

Physical vapour deposition duplex coatings for aluminium extrusion dies

Abstract Duplex coatings [plasma nitriding followed by physical vapour deposition (PVD) coating] have been given a lot of interest as surface treatment for forming and cutting tools in recent years. The advantage, as compared to conventional PVD coating, is a higher substrate hardness due to the nitriding prior to the PVD coating. The present investigation aimed at studying the effect of duplex treatment as compared to conventional PVD coating and nitriding, respectively, for aluminium extrusion dies. The tested surface conditions were CrN, TiN and TiAlN, all single-layered and duplex-treated. A salt-bath-nitrided specimen was investigated for comparison and reference purposes. Wear tests were undertaken using an extrusion simulation device. Prior to wear testing, coating properties were evaluated by scratch testing, surface roughness, hardness and residual stress measurements. Wear tests indicated that a PVD coating reduced the wear rate by about one order of magnitude as compared with the salt-bath-nitrided specimen. Duplex treatment resulted in a further reduction of wear rate by two to four orders of magnitude. TiAlN was shown to be the most wear-resistant of the tested coatings, both as a duplex coating and as a single layer. In addition, a field study with CrN duplex-treated extrusion dies was performed. The tools were worn by delamination of the coating followed by severe wear of the exposed substrate.

New physical vapour deposition coatings applied to extrusion dies

Abstract The bearing surface of dies for extrusion of aluminium is subjected to very difficult wear conditions. To minimize wear and thereby obtaining a longer service life, these dies are typically made of hot work tool steel and surface treated by nitriding. In the present study both commercial and experimental physical vapour deposition (PVD) coatings are evaluated in a device which simulates the bearing wear in an extrusion die. The tested surfaces include hot work tool steel, coated with TiN, CrN, (Ti,Al)N and TiB 2 , respectively. To simulate the extruded material an aluminium cylinder heated to 550°C is used as counter body in a block-on-ring configuration. For comparison, the tool steel was also tested in uncoated conditions. Prior to wear testing, the test surfaces were characterized by roughness, hardness, thickness and intrinsic stress. It is shown that coatings of CrN, and especially (Ti,Al)N and TiB 2 have the potential to effectively reduce the wear in extrusion dies. These coatings are mechanically, thermally and chemically stable when sliding against hot aluminium, in contrast with uncoated steel. It was also observed that the mechanical strength of the TiN coating gradually degenerated during this test, probably due to poor chemical inertness when exposed to hot aluminium.

Novel load-carrying layers to support low-friction PVD coatings

Five different materials have been studied with respect to their load-carrying capacity together with PVD coatings. The materials were mild carbon steel, hardened and annealed ball bearing steel, a thick electro-deposited Ni coating, and plasma-sprayed alumina coatings with and without the pores and cracks sealed with electro-deposited Ni. The two latter materials have not previously been studied in this respect. A WC-doped hydrogenated diamond-like carbon (Me-C:H) coating was deposited on each material. All materials were subjected to mechanical and tribological testing, including scratching, nanoindentation and pin-on-disc testing. The sealed plasma-sprayed alumina coating, as well as the unsealed, behaved surprisingly well as load carriers for PVD coatings, despite their porosity and low cohesion. The overall trend was that hard materials generally functioned better than soft ones. However, there is at present a small risk of chipping of the sprayed materials, which can be reduced by future process optimisation.