School of Mechanical and Manufacturing Engineering Dublin City University Surface Engineered Materials

Introduction The focus of this modules does not include paint, long established coatings such as those applied by electroplating, hot dipping (as used in galvanising) and mechanical plating. Most of these are for low load applications, or for decorative purposes (and are environmentally costly). We do focus on more recent developments such as thermal barrier coatings, high temperature corrosion and wear resistant coatings, and biocoatings.

Metallic Non-Metallic Chemical Conversion Polymer Glass Ceramic Oxide Anodizing Phosphate Chromate Vacuum Deposition Furnace Fused Chemical Vapour Deposition Hard Facing Vapour Deposition Miscellaneous Techniques COATINGS Coating deposition technologies, adapted from Bhushan & Gupta, (1991), & Stokes, ‘03.

Atomised Liquid Spray DIP Process Sol-Gel Fluidized Bed Brush, Pad, Roller Chemical Deposition Chemical Conversion Intermetallic Compound Spark Hardening Electro-Chemical Deposition Spin-on Hard Facing Vapour Deposition Miscellaneous Techniques Physical Vapour Deposition Chemical Vapour Deposition Evaporation Ion Plating Sputtering Welding Thermal Spraying Cladding Flame Electric Arc Plasma Arc Spray & Fuse Low Pressure Plasma Detonation Gun Electric Arc Plasma Arc Flame (HVOF) Deformation Diffusion Brazing Welding Laser Coating deposition technologies, adapted from Bhushan & Gupta, (1991), & Stokes, ‘03.

Increasing resistance to surface damage Creating a smooth, uncracked surface resistance to nucleation of cracks due to fatigue, contact loading or corrosion Increasing resistance to corrosion Increasing resistance to wear Increase the threshold value of the stress intensity factor

Chemical Compatibility Mechanical Compatibility- cohesion and adhesion. Deposition Process Compatibility – the process used cannot be such that it compromises the substrate properties (eg. by exposing it to too high temperatures) Component Geometry – some geometries cannot be coated by ‘line of sight’ techniques, and others cannot accommodate large sized components. Service Environment – the coating must function well within the service environment, and not be effected by contamination in process gasses, liquids. Repair requirements Selecting coatings

Some coatings and modified surfaces

High temperature coatings For many key engineering processes efficiency improves with temperature. This is the driving fact behind increasing operating temperatures for a wide range of engineering components. Limiting factor is lack of materials to operate at high temperatures. One method used to push up the temperatures by the use of high temperature coatings. Coating in this area is primarily concerned with protecting components from oxidation, corrosion and erosion by particle debris, thus prolonging their life. Traditionally the coating has developed independent of the substrate materials, but it is now recognised that as service conditions become more severe, that the two should be considered as a system.

Degradation Degradation modes for components in this service environment include low cycle thermo mechanical fatigue, foreign object damage, high cycle fatigue, high temperature oxidation, hot corrosion and creep. Damage to the coating itself can occur in one of two ways: surface damage, and diffusion based changes at the coating/substrate interface. The latter can compromise substrate properties, and deplete the coating of some elements. Can be difficult to evaluate interface damage.

3 types of high temperature coatings 1.Aluminide, 2.Chromide and 3.MCrAlY The substrate generally used with these coatings are Nickel based superalloys, however these are limited by their melting point for even higher temperature use. Ceramic intermetallic and refractory metals are candidates for replacement, but have very different mechanical, physical and chemical properties, will still need coating protection, and will be less tolerant of coating flaws. For this reason, research continues into the use of graded composition and multiplayer coating.

Laser modified surfaces Lasers or electron beams are used for rapid solidification surface modification of materials. This is done by scanning a high power beam over the material surface, to induce melting of a thin surface layer. Because of the high rate of energy delivery, this is a very efficient way of melting material, and very little of the energy is wasted in heating the substrate. Because the substrate remains ‘cold’, the melted material cools very rapidly when the energy input is removed. This rapid quenching infers the desired properties on the material.

Application of laser modified surfaces This technique has been applied to precipitation hardened nickel based superalloys, martensitically strengthened steels, and carbide dispersion strengthened alloys, and generates a refined surface microstructure which can considerably enhance component life. In 304 stainless steel laser glazing effects carbides at grain boundaries, thus improving resistance to stress corrosion cracking. In 614 Al bronze it homogenises the surface, improving its resistance to corrosion in chloride solutions. In high speed steels it generates a uniform fine distribution of hard carbide particles which improves cutting performance. An interesting area of current development is the use of laser glazing, in combination with powder or reactive gasses to make surface compositional changes as well as structural ones.

DLC Diamond, a crystalline form of carbon, has remarkable characteristics It is the hardest material known, has a high stiffness and strength, has high thermal conductivity and shock resistance, is chemically inert, and excellent infra-red transmission. In the form of a thin coating, diamond-like carbon uses many of these properties to the benefit of a component. While there are several processes used to generate such film, they all rely on bombardment of a substrate with carbon ions. Diamond like carbon refers to a mixture of amorphous and crystalline carbon phases, and its properties vary with deposition conditions. The films are hard, and generally have low coefficient of friction. They are chemically durable, and abrasion resistant. They may have high internal stress, which can limit thickness (which is of the order of 2-5  m).

Evaluation of Coating Properties 6.71 Å Graphite Diamond Fullerene C 60 Fullerene C 70 DLC Carbon C Diamond (sp 3 ) Graphite (sp 2 )

Application of DLC Protecting moving parts, components exposed to attack by oxygen or moisture, optics, optical devices, and in biomedical components. Tissue can adhere well to carbon implants, and in blood environment a protein layer forms which stops clotting at the carbon surface. A DLC coating on metal implant combines the strength of the latter, with biocompatibility of carbon.

Work on DLC at DCU ADHESION AND COHESION PROPERTIES OF DIAMOND-LIKE- CARBON COATINGS DEPOSITED ON BIOMATERIALS BY SADDLE FIELD FAST ATOM NEUTRAL BEAM SOURCE; MEASUREMENT AND MODELLING BY M. M. Morshed (B.Sc.Eng., M.Sc.Eng., PhD) NCPST

Experimental Procedure 1. Biomaterial  316L stainless steel (wt%: 0.03%C,18%Cr,10%Ni, 3%Mo, Fe (balance) Thickness: 8mm and 0.25mm, Diameter: 25mm  Co-Cr and Ti6Al4V alloys (wt%: 69%Co,25%Cr and 5% Mo) Thickness: 8mm and Diameter: 25mm  0.75mm Thickness Glass substrate 2. Sample Preparation  Grinding  Polishing (240, 600, 1200 grade emery papers  and 0.25  diamond polish)  Ultrasonic Cleaning 3. Saddle field source  Neutral beam deposition, energetic molecules  Also allows deposition on insulating substrates

5. Film Deposition Parameters  Process Gas: C 2 H 2 and C 2 H 2 +Ar gas mixture  Time: 1 hr.  Voltage: KV  Current: 0.6, 1A  Pressure:1.5x10 -3 to 4.8x10 -3 mbar Temperature  Etching temperature  Deposition Temperature 4. Argon Etching In-situ etching - Energetic argon atoms  Time: 0, 05, 10, 15 and 20 min.  Voltage: 1-1.7KV  Current : 0.6A, 1A  Pressure: 1.5x10 -3 to 4.8x10 -3 mbar

NEUTRAL BEAM FAST ATOM SYSTEM

Mechanical Characterization ADHESION TESTS A) Quantitative Adhesion Pull-off Adhesion Substrate Coating Stud Epoxy glue F B) Qualitative Adhesion Rockwell C Adhesion Normal load (1471N) Coated sample

Rockwell C Adhesion Better Adhesion HF1 or HF2 Medium Adhesion HF3 or HF4 Poor Adhesion HF5 or HF6

Residual Stress in Film Bending beam method STONEY EQUATION: z-axis After deposition Before deposition Deflection,  Distance, x-axis (  m)

Film Thickness SURFACE PROPHILOMETER