T. Bell

Plasma surface engineering of low alloy steel

Abstract The surface of low alloy steel (En40B) has been engineered in the plasma of a glow discharge via plasma nitriding and ion plating of titanium nitride (TiN) coatings on the nitrided substrates with the purpose of enhancing the surface properties and fatigue strength. The nitriding response of the steel has been accessed by the evaluation of phase composition, layer thickness, hardness profile, residual stresses and nitrogen and carbon distributions. The wear and fatigue characteristics of the plasma-nitrided steel have been investigated and simple models have been developed to describe the influence of such properties as depth and strength of the nitrided case on the fatigue limit and load-bearing capacity of the nitrided steel. In order to further improve the tribological properties and load-bearing capacity of the low alloy steel, a duplex plasma surface-engineering technique has been developed. This is achieved by plasma nitriding the steel first so as to produce a thick, strong subsurface and then depositing a thin, hard and wear-resistant TiN coating on the nitrided substrate by ion plating. Dry-sliding wear tests demonstrated that the duplex-treated steel, i.e. the TiN coating-nitrided steel composite, not only exhibited enhanced wear resistance over the as-nitrided steel (by a factor of 2–8) but also had much higher load-bearing capacity than the TiN coating on unnitrided steel. Optimization of the coating-substrate combination can be achieved by correct control of the plasma-nitriding, surface preparation and ion-plating processes.

Realising the potential of duplex surface engineering

Abstract We are now close to the new millennium and on the threshold of an era of rapid change. Limitations to the further advance of manufacturing industry in the 21st century are most likely to be surface-related. Many mechanical systems will operate under ever more severe application conditions, such as intensive loads, high speeds and harsh environments, in order to achieve high productivity, high power efficiency and low energy consumption. Consequently, many challenging complex design situations have emerged where the combination of several properties (such as wear resistance, load bearing capacity, and fatigue performance) are required. These new challenges can be met only through realising the potential of duplex surface engineering. Indeed, there are thought to be great technical and economic benefits available through the application of duplex surface engineering technologies in many new market sectors. The present paper is a synthesis of several strands of recent surface engineering research at the University of Birmingham, including the duplex ceramic coating-nitrided steel system and the duplex DLC coating–oxygen diffusion treated titanium system. The prediction of the performance of duplex systems based on advanced contact mechanics modelling is also discussed.

A numerical model of plasma nitriding of low alloy steels

Abstract A mathematical model has been developed to simulate the plasma nitriding process of low alloy steels, considering the simultaneous diffusion of nitrogen in ferrite, precipitation of fine scale alloy nitrides in the diffusion zone and development of γ′-Fe4N iron nitride layer on the surface. The present paper discusses various reactions occurring during the plasma nitriding process, mathematical formulations for various reactions to form the complete model of plasma nitriding, and a numerical solution to solve the model. Computation details and results for model Cr and Ti bearing steels and commercial steels En40B, En19 and Nitraps are also presented in the paper. It is shown that the model can predict the distribution of nitrogen in solution and in the form of alloy nitride precipitates, the iron nitride layer thickness developed during plasma nitriding, the incubation time for the formation of iron nitride layer and the nitride precipitation behaviour of various alloying elements in the steel. Application of the model to plasma nitriding of commercial low alloy steels demonstrated that the model describes the nitriding response of the steels to a quite satisfactory accuracy.

Finite element analysis of plastic deformation of various TiN coating/ substrate systems under normal contact with a rigid sphere

The elastic-plastic axisymmetric contact between a rigid ball and TiN coatings of varying thickness on various substrates has been simulated using the finite element method. Yield strengths and modulus values representing typical values for high-speed steel, titanium and aluminium alloys differentiated between the substrates. The effects of such important parameters as the thickness of the TiN coating and properties of the substrate material on the initiation and development of plastic deformation and the load bearing capacity of the TiN coating/substrate systems have been investigated. The results show that in most of the coating/substrate systems, plastic deformation is initiated in the substrate at the coating/substrate interface and plastic deformation does not initiate in the TiN coating until a large plastic zone has been developed in the substrate. Substrate strength and coating thickness have a significant influence on the plastic deformation behaviour and the load bearing capacity of the composite. In light of the finite element analysis results, the mechanisms of coating failure are also briefly discussed.

Enhanced corrosion resistance of duplex coatings

Abstract A series of electrochemical tests have been carried out to investigate the corrosion behaviour of physically vapour deposited TiN, CrN and (TiAl)N coatings on plasma nitrided En40B steel. Surface and subsurface characterisation before and after corrosion testing were performed using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). The experimental results indicate that all three duplex coating systems possess superior corrosion resistance over the individually plasma nitrided or PVD coated En40B steel, highlighting the importance of the iron nitride subsurface in determining the corrosion resistance of duplex coating systems. It is also demonstrated that among these three duplex coating systems, the corrosion resistance increases in the order of TiN/PN, CrN/PN and (TiAl)N/PN. The critical potentials corresponding to the onset of transpassive behaviour for both the CrN/PN and the (TiAl)N/PN duplex coating systems are above the practical potential range (

Examination of mechanical properties and failure mechanisms of TiN and Ti−TiN multilayer coatings

Abstract A number of studies have been carried out to establish mechanical properties of single and multilayer hard coatings. However, the mechanisms of deformation, cracking and delamination of coatings under ploughing and shear stress are not fully understood. A fractured cross-sectional specimen preparation technique through hardness indentation and scratch tests on hard coatings has been used in conjunction with high resolution SEM to observe deformation and fracture behaviour occurring as a result of these tests. TiN and Ti−TiN multilayer coatings were deposited on M2 high speed steel and silicon substrates using an unbalanced magnetron sputtering system. Hardness measurements and scratch tests were performed to monitor the mechanical properties. X-ray diffraction was used for phase identification. Coatings comprising fine columnar TiN behaved like closely congregated strong fibres: they were found to accommodate a large amount of ploughing and shear stress through densification and shear deformation. On increasing the load above a certain value, rupture of heavily deformed TiN initiated at defect locations and the cracks propagated and coalesced into macrocracks. When the applied load was increased to near the critical load, close packed columns separated from each other and detached from the substrate, resulting in total failure. For Ti−TiN multilayers, hardness and critical load are related to the different monolayer thickness of the Ti and TiN. The Ti layers dissipate most of the energy by means of shear deformation during the scratch test. At higher scratch loads, cracks occurred at Ti−TiN interfaces or at multilayer- substrate interfaces depending on the relative interface strengths. The influences of substrate hardness on the indentation crack pattern and scratch failure mechanism are also briefly covered in this paper.

Wear resistance of plasma immersion ion implanted Ti6Al4V

Abstract The plasma immersion ion implantation (PI 3 tm ) process has been employed in the treatment of the Ti6Al4V alloy in order to improve its notoriously poor tribological properties. In particular, this study was undertaken with a view to its potential application for the surface engineering of orthopaedic implants. PI 3 has been developed over recent years at the Australian Nuclear Science and Technology Organisation (ANSTO). The hybrid nature of this technique combines elements of both ion implantation and plasma nitriding, and has been shown to produce components with unique surface properties and optimum performance characteristics. A detailed study of the PI 3 process on the Ti6Al4V alloy has been undertaken. Treatment was carried out in a pure nitrogen atmosphere at temperatures of 350, 450 and 550 °C. In each case, specimens were treated for 5 h, with a high voltage pulse (typically 40 kV) applied directly to the workpiece. Wear resistance of the treated samples was assessed using a standard CSEM pin-on-disc wear machine, with a single crystal ruby ball as the contact tip. Glancing angle X-ray diffraction (GAXRD) was employed to determine the phases present in the surface modified layer. These findings were then compared to those achieved from parallel work with conventionally ion implanted and low temperature plasma nitrided samples. It was established that a high treatment temperature of 550 °C was necessary for substantial improvements in the properties of the Ti6Al4V material. Under these conditions the PI 3 technique promoted significant increases in Knoop hardness, and wear resistance an order of magnitude greater than conventional ion implantation. Wear rates were typically reduced by four orders of magnitude compared to those of the untreated Ti6Al4V. This is thought to be associated with the increased mobility of nitrogen in α -Ti at these temperatures, producing a deeper, hardened case. The presence of TiN was observed in the microstructure of PI 3 Ti6Al4V samples at all temperatures in the range.

Surface engineering design: modelling surface engineering systems for improved tribological performance

Rapid developments in surface engineering technologies have made available many surface engineering systems to combat diverse component degradation problems and to meet the ever-increasing demands for combined properties in modern machinery operating under ever more severe conditions. Designers thus have many surface engineering technologies to select from in the design of components. Therefore, design methodology is urgently required for surface engineering to assist engineers in their design of components with the surface and substrate as a system. The present paper attempts to highlight recent developments in this area and to discuss the philosophy behind building such a design methodology through several case studies, including a real-rough surface contact model to simulate the elastic contact behaviour of multi-layer systems and a finite element model to predict the plastic deformation behaviour of the surface, subsurface and interfacial region.

Combined Plasma Nitriding and PVD Treatments

SUMMARYA duplex surface engineering technique has been developed in an attempt to successfully apply thin hard ceramic coatings on commonly used low alloy steels. The low alloy steel (En40B) was first plasma nitrided so as to produce a relatively thick and strong subsurface layer, and then deposited with such ceramic coatings as TiN, CrN and (TiAl) by plasma assisted PVD. A series experiments have been carried out to investigate the structures and properties of the duplex treated steel. It is shown that such combined plasma nitriding and PVD treatments can produce a variety of coating/substrate interface structures and tribological properties. By proper process control, the resultant ceramic coating/nitrided steel composite possesses superior tribological performance over the individually plasma nitrided or PVD coated steel.

Towards the design of dynamically loaded titanium engineering components

Abstract Surface engineering research centred at the University of Birmingham has, for several years, been directed at producing wear-resistant surfaces on titanium alloy substrates in order to facilitate the use of titanium alloys in general engineering applications, usually as direct replacements for steel components. More specifically, bearings and gears have been identified as an area in which this could be accomplished, where a low coefficient of friction, low wear rate and load-bearing capacity are all necessary. The processes investigated have included plasma nitriding, physical vapour deposition, laser surface alloying and electron beam surface alloying and currently ion implantation and plasma immersion ion implantation. Some results from plasma nitriding and energy beam surface alloying processing and evaluation achieved to date are included which have culminated in the use of a duplex surface engineering process. This process has allowed a Tiue5f86Alue5f84V alloy (where the compositions is in approximate weight percent) to be tested at 1300 MPa maximum hertzian contact pressure and 50% slip. This represents an improvement of 1200% in contact pressure and 500% in slip ratio survival over untreated Tiue5f86Alue5f84V.