Developing an (Al,Ti)N x C y Interlayer to Improve the Durability of the ta-C Coating on Magnetic Recording Heads

In this study, a new surface pre-treatment technique has been developed to improve the durability of the ultra-thin tetrahedral amorphous carbon (ta-C) coating for magnetic tape drive read/write heads. In this technique, prior to the deposition of an 8-nm ta-C overcoat, a 2-nm thin TiN interlayer was deposited on the heads surface and bombarded with energetic Ar⁺ and C⁺ ions. X-ray photo-electron spectroscopy results revealed that this surface pre-treatment technique would lead to the formation of an atomically mixed (AlTi)N _x C _y interlayer which can chemically bond the interlayer to the overcoat and substrate. The effect of this atomically mixed interlayer on the wear resistance of the ta-C coating was investigated using ball-on-flat tests as well as a functional tape drive tester. According to the ball-on-flat test results, the application of the (AlTi)N _x C _y interlayer was able to improve the wear life of the ta-C overcoat by up to 3.3 times as compared to that of the conventional ta-C coating. The results of the wear tests in a real head/tape interface were in agreement with the ball-on-flat results, and showed that while the conventional ta-C film was completely removed from the head surface, the ta-C film with (AlTi)N _x C _y was able to protect the head surface for wear tests of about 1.6 million meters.     

Tribological Behaviour of SiC Particle Reinforced Al–Si Alloy

The purpose of this study is to explore the effect of SiC reinforcement along with immiscible element addition in spray formed Al–Si base alloy. The investigation is done for four different compositions, i.e., Al–Si base alloy, Al–Si/SiC, Al–Si–5Sn/SiC and Al–Si–10Sn/SiC composite. The dry sliding wear properties of base alloy and composites were investigated against EN 31 steel at five different normal loads (14.7, 24.5, 34.3, 44.1 and 53.9 N). The tests were carried out in dry sliding conditions with a sliding speed of 1.6 ms⁻¹ over pin-on-disc tribometer. Each composition is tested at four different temperatures 50, 75, 100 and 150 °C. To determine the wear mechanism, the worn surfaces of the samples were examined using scanning electron microscope (SEM). The composites emerge to be better wear resistant material than base alloy especially at higher loads. The optimum wear reduction was obtained in Al–Si–10Sn/SiC composite at all the different normal loads and temperatures.     

Chromium oxide coatings applied to magnetic tape heads for improved wear resistance

This paper reports on an investigation of the wear of chromium oxide based very thin films. Linear data tape Advanced Digital Recording (ADR™) heads coated with 20- and 40-nm thick chromium oxide films have been tested subject to temperature/humidity matrix of 10 to 40°C/10 to 80% in order to assess the wear behaviour of the coating as a function of environment. The tested heads were analysed at various stages of wear, by use of optical microscopy (OM), atomic force microscopy (AFM) and Auger electron spectroscopy (AES). The results show that the most severe damage occurs at the highest relative humidity (80% RH) and for a given humidity, at the lowest temperature (10°C). Inversely, stain transferred from the tape to the head surface predominates at the lowest humidity (10% RH). Stain therefore appears to protect the coating against wear with the degree of protection increasing with the temperature.The wear process differs according to the coating thickness. This is attributed to the location of the maximum subsurface stress with respect to the coating/substrate interface. It is shown that this maximum stress occurs below the interface for 20 nm and at the interface for 40 nm thick coatings. This correlates to different observed wear modes.     

High resolution observations of friction-induced oxide and its interaction with the worn surface

A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron, an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni₃Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni₃Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear.     

Formation and Oxidation of Linear Carbon Chains and Their Role in the Wear of Carbon Materials

The atomic-scale processes taking place during the sliding of diamond and diamond-like carbon surfaces are investigated using classical molecular dynamics simulations. During the initial sliding stage, diamond surfaces undergo an amorphization process, while an sp ³ to sp ² conversion takes place in tetrahedral amorphous carbon (ta-C) and amorphous hydrocarbon (a-C:H) surface layers. Upon separation of the sliding samples, the interface fails. A rather smooth failure occurs for a-C:H, where the hydrogen atoms present in the bulk passivate the chemically active carbon dangling bonds. Conversely, sp-hybridized carbon chains are observed to form on diamond and ta-C surfaces. These carbynoid structures are known to undergo a fast degradation process when in contact with oxygen. Using quantum-accurate density functional theory simulations, we present a possible mechanism for the oxygen-induced degradation of the carbon chains, leading to oxidative wear of the sp phase on diamond and ta-C surfaces upon exposure to air. Oxygen molecules chemisorb on C–C bonds of the chains, triggering the cleavage of the chains through concerted O–O and C–C bond-breaking reactions. A similar reaction caused by adsorption of water molecules on the carbon chains is ruled out on energetic grounds. Further O₂ adsorption causes the progressive shortening of the resulting, O-terminated, chain fragments through the same O–O and C–C bond breaking mechanism accompanied by the formation of CO₂ molecules.     

Dry Sliding Friction and Wear Properties of Al–25Zn–3Cu–(0–5)Si Alloys in the As-Cast and Heat-Treated Conditions

Dry sliding friction and wear properties of ternary Al–25Zn–3Cu and quaternary Al–25Zn–3Cu–(1–5)Si alloys were investigated using a pin-on-disc test machine after examining their microstructures and mechanical properties. An alloy (Al–25Zn–3Cu–3Si), which exhibited the highest tensile and compressive strengths, was subjected to T7 heat treatment. Surface and subsurface of the wear samples were investigated using scanning electron microscopy (SEM). The hardness and both tensile and compressive strengths of the alloys increased with increasing silicon content, but the trend reversed for the latter ones above 3% Si. It was observed that T7 heat treatment reduced the hardness and both tensile and compressive strengths of the Al–25Zn–3Cu–3Si alloy, but increased its elongation to fracture greatly. Three distinct regions were observed underneath the surface of the wear samples of the Al–25Zn–3Cu–3Si alloy. The formation of these regions was related to the heavy deformation of surface material and mixing, oxidation and smearing of wear material. Al–25Zn-based ternary and quaternary alloys in both as-cast and heat-treated conditions were found to be superior to SAE 660 bronze as far as their mechanical and dry sliding wear properties are concerned.     

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACT

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.     

Effect of temperature,humidity and crystal directions on wear of rotary heads

In the advanced tape drive systems which achieve a higher recording density, the wear of rotary heads should be decreased to obtain a higher reproduced output signal by decreasing the ‘Gap depth’ of heads, keeping enough life time. By using the heads made by two kinds of crystal directions of Mn–Zn ferrite and metal evaporated tapes with DLC coating, temperature and humidity dependences of head wear are investigated. The wear rates of both crystal direction heads increase with the decrease in temperature. The wear rate of a (110) head increases very quickly at low temperature, compared with a (100) head. These temperature dependences of head wear are caused by the temperature dependence of the elastic modulus of the tape and the temperature dependence of the friction coefficient between the tape and the head.     

CrCN/CrN+ta-C multilayer coating for applications in wood processing

The paper presents mechanical and tribological properties of CrCN/CrN and CrCN/CrN+ta-C multilayer coatings. Tetrahedral carbon (ta-C) layer formed using the pulse cathodic arc evaporation method are characterised by high hardness –45 GPa, very low friction coefficient—below 0.1 and a low wear rate −1.3×10⁻¹⁷ m³N⁻¹ m⁻¹ providing promising application perspectives.Three sets of tools—planer knives for cutterheads were tested: uncoated (as reference), tools with a CrCN/CrN coating and tools with CrCN/CrN coating with additional friction-reducing tetrahedral carbon (ta-C) layer. The results of investigations indicate that the “tool life” depends on the type of coating and machining conditions. The blades covered with CrCN/CrN multilayer coating after machining of dry, seasoned pine timber showed a twofold increase of durability, and knives covered with CrCN/CrN+ta-C multilayer coating were characterised further by about 15% higher durability. Durability of knives tested in the course of rounding of wet pine timber, despite relatively high depth of machining was improved and for cutters with a CrCN/CrN coating increased more than twice, while the use of the additional ta-C layer on the multilayer coating improved durability by almost 5 times.     

Reciprocating Sliding Wear Performance of Hard Coating on Modified Titanium Alloy Surfaces

The high strength, low weight, and outstanding corrosion resistance properties possessed by titanium alloys have led to a wide range of successful applications in aerospace, automotive, and chemical industries and in power generation. Titanium alloys are characterized by poor wear resistance properties and their utilization has been excessive in nontribological applications. Surface texturing is a well-known and effective means of surface modification to improve the tribological properties of sliding surfaces. In the present work, modification of titanium alloy surfaces (Ti6Al4V) was done by lapping and laser surface texturing. The wear-resistant coating, AlCrN, was applied over the modified titanium alloy surfaces, with and without a chromium interlayer. Linear reciprocating sliding wear tests were performed with ball-on-flat contact geometry to evaluate the tribological performance of the coated alloy. The tests were performed under different normal loads for a period of 105 cycles at a frequency of 5 Hz. The friction force between the contact pair and displacement of the ball were simultaneously observed using a force transducer and laser displacement sensor. Optical microscopy was used to quantify the wear volume by measuring the wear scar diameter on both the specimen and the counterbody. Scanning electron microscopy (SEM) was employed to study the morphology of the wear scar. The characteristic behavior of the AlCrN coating such as bonding strength, wear volume, wear rate, and coefficient of friction with the chromium interlayer was evaluated and compared with the coating directly applied over the substrate. The coating on the textured surface, with the chromium interlayer showed better tribological performance.     

The measurement of in-situ wear of video heads

A method of measuring in-situ wear of video heads which determines the variation in wear along the surface of the video head is described. Knoop diamond indentations are positioned along the head, either side of the magnetic gap, enabling wear rate to be monitored at several positions. Virgin tape is run over the heads in the form of standard E180 cassettes and the wear occurring at each indentation position is derived from the reduction in length of the main diagonal. Wear rate is plotted against the position of the indentations. This describes the variation in wear along the head and therefore the conformation of the tape to the head. The technique has been used to study the wear of video heads with three types of tape sample: two iron-oxide-based formulations, with either Al₂O₃ or Cr₂O₃ added as head cleaning agents, and one chromium-dioxide-based tape. Little difference was found between the performances of the iron oxide tape samples. The chromium dioxide sample, however, produced wear some five to six times greater than the iron oxide tapes.     

On the importance of elasticity of materials in the wear of magnetic heads

An analysis of deformations of magnetic tape extended on a magnetic head is presented. The tape deformations are regarded as proportional to contact pressure exerted by the tape, which is of great consequence in the wear of magnetic heads sliding against such tapes. To calculate the distribution of tape deformations, the theory of thin elastic shells and the theorem on minimum potential energy are used. From comparison of these distributions with experimentally obtained surface profiles of worn heads, it is concluded that the wear process of magnetic heads is by nature connected with the elasticity of the materials both of tape and of head.     

Improvement of the wear behavior of stircast Al–Si–Pb alloys by hot extrusion

The wear behavior of as-cast and hot extruded Al–Si–Pb alloys were investigated under dry conditions using a pin-on-disc type wear testing machine. The results show that the microstructure and mechanical properties can be greatly improved and porosity can be significantly decreased by hot extrusion. These factors contribute to great increase in wear resistance of hot extruded Al–Si–Pb alloys. Optical observation and X-ray photoelectron spectroscopy (XPS) analysis reveal the almost constant wear rate at mediate load levels. Better resistance to seizure for Al–Si–Pb alloys with more than 15 wt% lead are due to a film of lubricant covering almost the entire worn surface. This film is a mixture of different constituents containing Al, Fe, Si, O and Pb.     

Angstrom Scale Wear of the Air-Bearing Sliders in Hard Disk Drives

We utilize thermal fly-height control (TFC) technology to perform in situ measurements of carbon overcoat wear at the angstrom level at the read–write area of magnetic recording heads. We also study the durability of the molecularly thin lubricated disk surface. Experimental findings reveal a linear relationship between the quantified carbon wear depth on the flying head versus the head–disk contact level produced by the TFC power. It is demonstrated that this method can serve as a measurement and probing technique of wear resistance for different types of lubricants. Lubricants possessing more polar hydroxyl end-groups and less mobility tend to show a superior surface stability under head–disk contacts, but raise concerns on head carbon overcoat wear.     

Tribological studies of automotive piston ring by diamond-like carbon coating

Increasing environmental awareness and demanding low energy consumption are of the top priorities for future vehicles manufacturing companies. This can be achieved by reducing wear and friction of engine components, so that its efficiency and lifetime can be increased. Surface treatments and coatings contribute to a better lubrication with oils and can participate significantly in achieving these goals. In this paper, diamond-like carbon (DLC) coating has been incorporated to the vehicle piston rings with different RF powers using magnetron sputtering method. The tribological properties like wear and coefficient of friction have been analysed using Pin-on-Disk tribometer. Micro-hardness and nano-hardness of the coated piston rings were characterized by micro-indentor and nano-indentation processes. Surface microstructure and elemental compositions were observed using Scanning Electron Microscopy. Experimental results demonstrated that the DLC coating shows lower wear and friction under similar operating conditions as compared to uncoated piston rings. Thus, usage of DLC coating has enhanced the engine life time. Silicon interlayer has also been applied between nitrided piston rings and DLC layer in order to have better coating adhesion. The properties of the interlayer are not studied but usage of it is found to protect DLC coating from delamination.     

Microstructure characterization of advanced protective Cr/CrN+a‐C:H/a‐C:H:Cr multilayer coatings on carbon fibre composite (CFC)

Studies of advanced protective chromium‐based coatings on the carbon fibre composite (CFC) were performed. Multidisciplinary examinations were carried out comprising: microstructure transmission electron microscopy (TEM, HREM) studies, micromechanical analysis and wear resistance. Coatings were prepared using a magnetron sputtering technique with application of high‐purity chromium and carbon (graphite) targets deposited on the CFC substrate. Selection of the CFC for surface modification in respect to irregularities on the surface making the CFC surface more smooth was performed. Deposited coatings consisted of two parts. The inner part was responsible for the residual stress compensation and cracking initiation as well as resistance at elevated temperatures occurring namely during surgical tools sterilization process. The outer part was responsible for wear resistance properties and biocompatibility. Experimental studies revealed that irregularities on the substrate surface had a negative influence on the crystallites growth direction. Chromium implanted into the a‐C:H structure reacted with carbon forming the cubic nanocrystal chromium carbides of the Cr₂₃C₆ type. The cracking was initiated at the coating/substrate interface and the energy of brittle cracking was reduced because of the plastic deformation at each Cr interlayer interface. The wear mechanism and cracking process was described in micro‐ and nanoscale by means of transmission electron microscope studies. Examined materials of coated CFC type would find applications in advanced surgical tools.     

Dry Sliding Wear Behavior of Zircon Sand Reinforced Al–Si Alloy

The emerging demand of light weight alloys and composites for the engineering and structural applications leads to explore the possibility to develop new techniques to achieve materials of high performance. In the present study, Al–Si/zircon sand reinforced composite has been developed via spray forming technique. Dry sliding wear behavior of as cast Al–Si base alloy and spray formed Al–Si/zircon sand reinforced composite containing 8% V _f of zircon sand has been analyzed. An isotropic wear property of spray formed composite has been checked by selecting the spray formed preform in both horizontal and vertical sections of deposit. The wear tests which were carried out at loads of 14.7, 24.5, 34.3, 44.1, and 53.9 N have shown that spray formed composite is more wear resistant in comparison to the cast Al–Si alloy. Moreover, wear coefficient in case of composite is also found to be lower than base alloy. Optical and scanning electron microscopies have been carried out to furnish a suitable explanation for observed wear behavior of composite and alloy.     

Antifriction and Wear Characteristics of Electrolessly-Deposited Ni–P with PTFE Composites

This aim of this study was to investigate the tribological properties of a self-lubricating Ni–P–polyfluorotetraethylene (PTFE) composite coating prepared by the electroless plating method. The effects of PTFE contents in the coating, load and rotation speed on the tribological behaviors were evaluated using a ring-on-disk wear machine. The results show that there was a distinct decrease in the average value of the friction coefficient from 0.33 to 0.12 at 70 N with an increase in PTFE content from 4.2 to 15.2 wt%. The coating of Ni–P–4.2 wt% PTFE had good antifriction and wear properties at a load of 30–70 N, and that of Ni–P–10.6 wt% PTFE had passable wear resistance and better antifriction at <50 N. Antifriction and wear mechanisms of Ni–P–PTFE are discussed in detail based on the results from micrograph and element analyses of the worn surface, subsurface stratum and wear debris analysis by scanning electronic microscopy (SEM), and energy-dispersive X-ray analysis (EDAX), respectively. The lubricating film (LF) generated during wear played a key role in the antifriction effect, which in turn was dependent on the integrity and thickness of the film determined by the PTFE concentration and wear conditions. The formation, fracture, and delamination course of the LF during wear were also analyzed and characterized.     

Effect of oil additives on the durability of hydrogenated DLC coating under boundary lubrication conditions

Diamond-like carbon (DLC) coatings have became accepted non-ferrous coatings for automotive tribo-components as they offer excellent tribological properties resulting in improved fuel economy and reduced dependence on existing lubricant additives which can be harmful to catalytic converters and ultimately to the environment. Obtaining optimum durability (wear) as well as high fuel economy (low friction) using DLC-coated parts relies in part on the compatibility between surface and lubricant additives. The objective of this study is to understand the role of friction modifiers and antiwear additives on the durability of DLC coating under boundary lubrication conditions. Experiments were performed using a pin-on-plate tribotester using plates coated by 30 at.% hydrogen containing DLC (HDLC) sliding against cast iron (CI) pins. The physical observation of the wear scar, formed on the HDLC coating by low friction and/or antiwear additives, was performed using both optical and scanning electron microscopes. X-ray photoelectron spectroscopy analysis was performed on the tribofilms to help to understand the tribochemical interactions between oil additives and the HDLC coating. Based on the physical observations and tribochemical analysis of the wear scar, the mechanisms of failure/wear of the HDLC coating are proposed and the requirement for designing optimal additive packages for the HDLC coating is discussed.     

Correlation between the characteristics of the thermo-mechanical mixed layer and wear behaviour of Ti–6Al–4V alloy

The sliding friction and wear behaviours of Ti–6Al–4V alloy were investigated under dry sliding wear conditions. The wear tests were carried out on a pin-disc tribometer at sliding speeds from 30 m/s to 70 m/s and at contact pressure ranging from 0.33 MPa to 1.33 MPa. Pins of the Ti–6Al–4V alloy are used in both solution treated and aged conditions. The objective of the study is to understand the influence of thermo-mechanical mixed layers (TMML), which form on the surface of the worn material during the course of the wear test, on the friction and wear behaviour. Detailed characterization of the TMML was carried out using SEM, EDS and micro-hardness testing in order to understand the influence of test velocity and contact pressure on the composition, hardness and thickness of the TMML formed. The influence of the TMML on the friction and wear behaviour was also studied. On the basis of the above characterization, it was demonstrated that the observed friction and wear behaviour of Ti–6Al–4V alloy can be best understood in terms of the formation and fracture rate of the TMML rather than the bulk properties of the material.