C. Subramanian

On the design of coating systems: Metallurgical and other considerations

Abstract Coatings and surface treatments are widely used in combating corrosion and wear. In particular thin hard coatings of borides, carbides, nitrides and oxides - both singly and in combination - have been synthesised by physical and chemical vapour deposition (PVD and CVD) methods, characterised and evaluated for possible applications in manufacturing industry. Many such coatings in both single and multilayer formats are now available commercially eg. cutting tools coated with TiN, TiAlN, TiC/Al 2 O 3 /TiN, etc.. To progress further in this direction there is a need to understand more fully the fundamentals of various material strengthening mechanisms and their relation to wear resistance in coatings, as distinct from bulk monolithic materials. This paper reviews the mechanisms of wear and degradation in several manufacturing scenarios (especially metal cutting and metal forming), and considers the available published correlations between wear resistance and microstructure, hardness, toughness and adhesion of coatings. Based on the knowledge of strengthening mechanisms in bulk materials, various possible ways to strengthen and improve coatings are discussed. A main focus of the paper is to consider and review the approach to, and rationale for, the selection of various possible coating materials - type and nature (eg. composition, and thickness) - for multilayer and composite coating systems.

Observations on the structure, hardness and adhesion properties of a selection of multicomponent refractory element nitride coatings

Abstract In recent years, the introduction of titanium-based nitride and carbide coatings has led to a marked improvement in the tribological behaviour and service life of many engineering components such as cutting tools and metal forming dies. Although TiN performs satisfactorily, increasing demands from manufacturing industries have driven the development of thin coatings to optimised performance. Coatings are being sought with, inter alia, improved hardness and wear resistance. In this context, attention has been focussed on a number of advanced multi-component nitride coating systems based on, or incorporating, refractory element additions. Thus the use and incorporation of refractory and semi-refractory elements such as Al, Zr, Cr or V promises to provide coatings with higher hardness and improved wear resistance due to the formation of novel complex structures. In the present investigation, a wide selection of single and multi-component coatings (TiN, CrN, ZrN, TiAlN, TiAlVN, TiCrN, TiZrN and TiCN) have been deposited onto high speed steel substrates, using the technique of unbalanced magnetron sputtering. This paper presents data on the structure, hardness and adhesion properties of these coatings. The implication of these results is then discussed.

Bending fatigue and contact fatigue characteristics of carburized gears

Abstract In order to assure reliability, the manufacture of gears for power transmission in high load bearing applications requires precise control of a wide range of processing parameters during manufacture, aimed at achieving specific material properties. Process selection and control in forming and heat treatment operations plays a crucial role in gear production, and must start at the design stage. Material selection, choice of forming processes, thermochemical and heat treatment operations, surface finish and condition will ultimately determine the integrity and performance limits of the gears. An ongoing research programme by Birrana Engineering, Adelaide, and the University of South Australia involves the evaluation of materials, process route and process control used in the manufacture of a range of gears and relates these factors to fatigue resistance, with the aim to reduce production costs and improve gear quality and reliability. The results of single tooth bending fatigue, and contact fatigue tests on production gears manufactured from EN39B steel (steel A), and experimental steels B, C and D (heat treatment details restricted for commercial reasons) are presented and discussed.

Influence of substrate roughness on the scratch adhesion of titanium nitride coatings

Abstract Hard coatings such as titanium nitride are currently used in many applications, e.g. metal cutting, metal forming, plastic moulding and decorative fields. For a coating to be successfully functional, it should possess good adhesion to the substrate. The adhesion of a coating depends on factors which include, inter alia , surface preparation methods, affecting surface cleanliness, condition and roughness. The present study describes some results concerning the influence of surface roughness on the adhesion of titanium nitride coatings to high speed steel substrates. A scratch adhesion measuring unit was used to evaluate the adhesive strength of the coatings. Scanning and optical microscopes were employed to characterise the scratched surfaces. It has been shown that the critical load to failure decreases with increasing substrate roughness. The significance of these observations is considered especially in the broader context of quality assurance and manufacturing efficiency.

The role of refractory element based coatings on the tribological and biological behaviour of orthopaedic implants

Abstract In the area of orthopaedic implants, particularly hip joint replacements, only three groups of metals and alloys are considered suitable as load - bearing implant materials, namely Ti ue5f8 Ti alloys, Co ue5f8 Cr alloys and 316L stainless steel. As a result of problems, largely associated with failure of the anchoring poly methyl methacrylate (PMMA) bone cement, the survival of present-day load - bearing prostheses cannot be guaranteed beyond 10 to 12 years. This limited lifespan has essentially limited joint replacements to the elderly. However, the increased number of younger patients requiring replacements has accentuated the need for joint prostheses with improved performance and longevity. The introduction of joint prostheses however with significantly enhanced durability in turn accentuates problems associated with implant loosening, wear, corrosion and, in particular, biological incompatibility. At present, very few monolithic materials are capable of satisfying all service requirements. The solution may lie in the use of coating technology, creating new materials / surfaces with unique combinations of properties that cannot be achieved from the use of monolithic materials alone. The first part of this paper reviews the types of coatings that have been used, or are currently being considered as implant materials, to include porous metallic coatings, “bio-active” (hydroxyapatite, bioglasses) and “bio-inert” (alumina) ceramic coatings. Their advantages and limitations will be discussed. The second part will focus on recent developments in the use of refractory element based coatings, with specific reference to work conducted on sputtered niobium and tantalum. Possible avenues for research to include advanced coating (multilayered / multicomponent) systems, surface treatments and duplex engineering will be reviewed.

Performance evaluation of TiN-coated twist drills using force measurement and microscopy

Abstract In the present investigation, the performance of TiN-coated M2 high speed steel drills has been systematically evaluated by drilling a plain carbon steel workpiece. Tests were also performed with uncoated drills for comparison purposes. During the course of testing torque and thrust forces were recorded continuously. A scanning electron microscope was used to elucidate the possible wear mechanisms of worn drills. These studies indicate that various modes of wear are operative in the coated and uncoated tools. The effects of the surface finish and geometry are discussed in the broader perspective of quality assurance needs for coated tools.

Surface engineering: An enabling technology for manufacturing industry

Abstract Surface Engineering (SE) - a term practically unknown even a decade ago - can be described as an enabling technology, applicable to a wide spectrum of industrial sector activities. The subjects emergence as a distinct, integrating discipline, encompassing many areas of engineering, physics and materials science, has been greatly influenced and driven by recent major developments in coating and treatment process technologies and by the realisation that the surface is the most important part of any engineering component. It is from surface-initiated effects that most components fail. Technological advance is often constrained by surface demands. Commonly surfaces with enhanced friction, wear and corrosion resistance are required. The role of SE to satisfy these requirements, involving the design and creation of novel coating systems, is illustrated. The need for appropriate quality assurance (QA) and quality control (QC) procedures for advanced surface coatings is indicated.

Properties and characteristics of advanced tribological surface coatings and the assessment of quality-for-performance for enhanced manufacturing efficiency

Abstract A great and increasing variety of advanced surface coatings produced by many different process technologies are becoming available to enhance manufacturing efficiency via, inter alia , increasing tool and die life (reducing down-time), permitting faster, deeper cuts, reducing the number of separate step operations, effecting improved surface finish and better tolerances, and reducing power comsumption. The rapidly-developing variety and sophistication of such surface engineering practices - for example multilayer format coatings produced by combination technologies - increasingly demand re-assurance as to quality. Here a basic problem is to be able to confidently associate the characteristics of a coating - that is, its composition, structure, morphology, surface finish, thickness, adhesion, cohesion, hardness, internal stress level etc. - with performance and reliability in typical manufacturing operations. These matters are reviewed and discussed.

The efficiency of uncoated and coated tool systems in the machining of low carbon steel assessed through cutting force measurements

Experiments have been conducted on uncoated and coated carbide and ceramic lathe tool inserts of differing geometries at various speeds and feed rates. Each insert was tested for 20 s during which the cutting forces were measured with a Kistler three-component piezoelectric dynamometer and evaluated using a computer-based system. The experiments were carried out so that all variations in the tool forces were affected only by the mechanical properties of the tools. The effects of systematically varying coatings and tool geometries on cutting forces were analysed at specified time intervals. It has been established that the geometry of cutting inserts has a significant effect on the cutting force components. Uncoated carbide inserts produced forces about 15% higher than ceramic inserts, while 10% variations in cutting force component values were obtained from coated carbide inserts and ceramic inserts with different geometries. It was also found that in unstable cutting conditions ceramic inserts were associated with higher variation in force component values than carbide inserts. The prospects for the use of these kinds of data for adaptive control and tool design in quality manufacturing are discussed.

Conditions prevailing in the carburising process and their effect on the fatigue properties of carburised gears

Abstract Gears used in power transmission in mining equipment are subject to severe operating conditions. The performance limits of the gears are influenced by material selection, choice of forming processes, heat treatment, surface treatment and surface finish. Carburised steels are used widely for gears which undergo cyclic loading (fatigue). In this paper, three steels, viz., EN39B steel, steel X and steel Y, were treated differently (heat and surface treatments) and evaluated in a single-tooth bending test rig. Fractographic observation, using optical and scanning electron microscopes, showed that inclusion-initiated fatigue cracks initially grew in transgranular mode, reaching a critical size before rapid fracture occurred through the carburised case in a brittle manner, resulting in mixed intergranular/transgranular cracking. In EN39B and steel X, fatigue cracks were observed to have initiated from sub-surface MnS inclusions whereas cracks in steel Y were sub-surface initiated. Subsequent failure of the core material occurred in a ductile mode. An important outcome of this study is that a cheaper steel (steel Y) with a combination of treatments (carburising and glass-bead peening) has out-performed the other two steels. The results are presented and discussed.