Evaluation of erosive wear resistance of TiN coatings by a slurry jet impact test

In this paper, it is proposed to use a new type of solid particle impact test (slurry jet) to swiftly evaluate wear properties of thin, single layered or multilayered coatings. By the slurry jet, 1.2 μm alumina particles were impacted at high velocity perpendicular to thin PVD coatings of TiN deposited on high speed steel substrate materials under various substrate temperatures. Since the coatings have a much higher wear resistance than the substrate material, the wear rate increases significantly to the higher level of the HSS material when the coatings are penetrated. This is utilized in the quantification of the assessment of coating wear. A ranking of wear resistance and correlations to the coating surface hardness measured by nano-indentation tests, and coating morphology and structures are given and discussed. The TiN deposited under the highest substrate temperature proved to have the highest wear resistance although it had a relatively low hardness. The wear rate of the TiN coatings varies with the orientation of grains, that is, the {1 1 1} orientation that dominates for the high temperature deposition shows a higher wear resistance than the {1 0 0} orientation, which corresponds with the cleavage fracture behavior. Thus, it can be recommended as a screening test when evaluating coatings and coated materials.     

Direct current bias applied to hot flame diamond deposition produces smooth low friction coatings

As-deposited diamond coatings generally have a high surface roughness which results in a high friction coefficient and extensive wear of the counter material in sliding contact. Therefore several methods for smoothening diamond coatings have been proposed, such as laser polishing, molten metal etching, thermochemical polishing and mechanical polishing. All these methods have some disadvantage e.g. long processing time or high processing temperature. Furthermore, they are all post-deposition treatments i.e. the manufacture of these coatings requires at least two processing steps, deposition and smoothening. With the present method which combines d.c. bias with hot flame diamond deposition, a smooth diamond surface is produced during the actual growth of the film. No post-deposition treatment is necessary. The surface roughness is not dependent on the coating thickness which means that thick coatings with smooth surface can be produced. In fact, the method has a smoothening effect, i.e. rough surfaces can be made smooth. The method is comparable to conventional hot flame deposition of diamond as to growth rate and cost of producing the coatings. The coatings have a nano-crystalline structure and a surface roughness of Ra = 25 nm, and result in a friction coefficient of 0.1 or less in dry sliding and about 0.05 in water-lubricated sliding against cemented carbide. Their wear resistance is virtually the same as that of conventional diamond films.     

Wear study of field worn clutch actuators and evaluation of a model test

Pneumatic clutch actuators, employed in trucks, have been investigated. The surfaces of the inside of the anodised aluminium cylinders, of the lip seals and of the guiding rings have been studied by SEM and optical microscopy. For most of the actuators no significant wear was revealed. However, one actuator, with leakage problems, was severely worn. An extensive amount of scratches was found on the cylinder surface, the thickness of the guiding ring had decreased and large parts were missing from the lip seal. A possible explanation is that particles have entered the system thus altering the contact conditions. A test setup was developed to investigate how particles present in the system affect the wear. A part of the piston, with lip seal and guiding ring, slides against a part of the anodised aluminium cylinder in a back- and forward motion. Pure silicone grease lubricated tests resulted in no wear. Tests with added dust particles resulted in distinct scratches on the aluminium surface and embedded particles in the guiding ring and the lip seal. These tests provide results in good agreement with the wear revealed in the investigations of used actuators and support the theory that wear is caused by particles.     

Intermittent metal cutting at small cutting depths—1. Dead zone phenomena and surface finish

Chip formation in intermittent metal cutting at small cutting depths was investigated by single edge experiments. Single cutting strokes were performed in a modified Charpy pendulum tester which offers force measurement, accurate selection of cutting speed and feed in the ranges typical of many intermittent high speed steel (HSS) tool operations. The pendulum is also provided with an excellent quick-stop mechanism.

The cutting performance of HSS tools in three widely used steel grades (including one plain carbon, one quenched and tempered and one austenitic stainless steel) was studied. A number of double rake micro geometries, with primary rake angles ranging from +20° (parrot bill) to −60°, all with a prepared 0.1 mm wear land were tested. The performance of the different edge geometries was investigated with respect to class of dead zone developed on the cutting edge, and its relation to chip curl and finish of the cut surface. The results are visualized in a dead zone map. The influence of cutting length, cutting speed, cutting depth and TiN-coating was treated specifically.

Among the most important observations were:

• the micro geometry of the edge influences the dead zone formation mechanism and hence the class of dead zone,

• the surface finish is strongly dead zone class dependent,

• the chip curl is determined by edge micro geometry and dead zone class.

The relationships between the varied parameters, generated dead zones and resulting cutting forces are presented in part 2 of this paper.     

Intermittent metal cutting at small cutting depths—2. Cutting forces

Cutting forces in intermittent metal cutting at small cutting depths were investigated by single edge experiments. Single cutting strokes were performed in a modified Charpy pendulum tester which offers cutting and thrust force measurement and accurate selection of cutting speed and feed in ranges typical for many intermittent high speed steel (HSS) tool operations.

The cutting performance of a number of double rake HSS edges, with primary rake angles ranging from +20° (“parrot bill”) to −60°, all with a preground 0.1 mm flank length were tested in two steel grades (one plain carbon and one austenitic stainless). Some of the edge geometries were tested also in TiN coated condition. The relative performance of the different edges was investigated with respect to specific cutting and thrust forces. The influence of cutting length and depth, edge micro geometry, TiN coating and cutting speed is discussed specifically.

Among the most important observations were:

• The cutting and thrust forces at a fixed cutting depth may change significantly during the short (25–30 mm) cuts.

• The chamfer formed by a double rake geometry with negative primary angle increases the forces.

• For these chamfered tools the forces increase linearly with the projected flank length. TiN coating increases rather than reduces the forces during these short cuts.

The relationships between the varied parameters and chip formation phenomena like dead zone formation, chip curl and surface finish were presented in part 1 of this paper.     

A specimen preparation technique for transmission electron microscopy of surface layers

A TEM specimen preparation method is described, with the aid of which electron transparent foils can be obtained across the external surface of a specimen. After careful pre-treatment, steel specimens have been electrolytically coated with nickel. Conventional thinning in a plane cutting the substrate-coating interface, gave thin foils displaying the internal structure as a function of depth under the initial free surface. The method has also been applied to minute metal particles, of dimensions too small to allow manipulating and foil preparation by conventional methods. Image examples are shown, and the applicability of the method is discussed.     

Experiences from scratch testing of tribological PVD coatings

The use of PVD coatings in tribological applications becomes more and more widespread. Thus also the need to fully understand the relationships between the intrinsic properties of the coating, the properties of the coating/substrate composite and the tribological performance of the composite in different tribological systems becomes increasingly pressing. One of the tools available for tribological characterization of coatings and coating/substrate composites is scratch testing. In the current paper, Uppsala University presents a selection of results from many years of scratch testing of PVD coated components. Applications range from adhesion assessment and coating quality determination to estimation of coating fracture resistance. Examples in the form of scratch studies of PVD coatings on various high speed steels and tool steels - including failure mode anaiysis in situ SEM - are given.     

Comparison of two test methods for evaluation of forming tool materials

Two test methods often used to simulate the tribological performance of forming tool materials are compared in this investigation through an evaluation of the friction and wear properties of four tool steels in dry sliding. One test (slider‐on‐flat‐surface (SOFS)) utilises a vertical disc sliding on a horizontal flat test surface, and the other (load scanner (LS) tests) utilises two crossed cylindrical rods. The test conditions were selected as equal as possible for the two tests, and the following conclusions are made. Somewhat unexpectedly, the friction and wear results differed substantially between the two tests. The SOFS test showed a better potential to evaluate wear resistance since one test sample is in continuous contact with the other. The LS test can generate higher contact pressures since the two rods contact each other in an unworn condition throughout the whole test stroke. LS indicate that two hard and smooth tool steels tested against each other generally give low friction and good galling resistance. The two hard couples tested sustained the highest loads without any detectable surface damage. For the same combinations of hard steels, SOFS gave a higher friction due to the wear of the disc. The carbides in the disc material resist wear better than the matrix and will consequently wear the disc by abrasion, which adds to the friction. The above conclusions are drawn from a rather limited examination using only one set of test parameters. In fact, the two tests are both very flexible as to the way they can be used, and they both have advantages and limitations in tribological studies. They should rather be considered complementary than competitive. Copyright © 2008 John Wiley & Sons, Ltd.     

Low-friction carbon-rich carbide coatings deposited by co-sputtering

Low-friction coatings are used more and more frequently, particularly in situations and applications with insufficient or no lubrication. A good example of such coatings is amorphous carbon, which is produced both in pure form (a-C:H) and doped with metal (Me-C:H). The knowledge of what actually occurs when one metal in a Me-C:H coating is exchanged with another has so far been rather limited. Also, when producing these films hydrogen is incorporated in the substrate as well as in the film, which can be detrimental to the overall properties.Here, a newly adopted co-sputtering technique, utilizing a carbon target partly covered by metal-foil strips, was used to deposit non-hydrogenated carbon coatings alloyed with Ta, W and Zr on ball-bearing steel (BBS) substrates. The metal content varied between 0 and 41 at.%, and the resulting films were analyzed with respect to phase composition and textures, chemical composition, microstructural morphology, as well as mechanical and tribological properties. All alloyed coatings displayed a nanocomposite microstructure, with 3-6 nm metal-carbide crystallites embedded in a matrix of amorphous carbon. The amount of metal-carbide phase increased with increasing amounts of metal which led to a large increase in hardness and elastic modulus. An increased metal content did however not affect the carbide size to any notable extent. Ball-on-disk tests show that metal additions cause a sharp drop in friction coefficient from 0.21 to about 0.05, depending on the metal used. This is however accompanied by an increase in wear rate. The coating best combining low friction and low wear rate was alloyed with 20 at.% Ta. Best possible protection of the counter surface was offered by coatings containing 30 at.% Ta or more.