Surface Engineered Materials School of Mechanical and Manufacturing Engineering Dublin City University 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. COATINGS Non-Metallic Metallic Chemical Conversion Oxide Anodizing Phosphate Glass Ceramic Polymer Vacuum Deposition Furnace Fused Chemical Vapour Deposition Chromate Vapour Deposition Hard Facing Miscellaneous Techniques Coating deposition technologies, adapted from Bhushan & Gupta, (1991), & Stokes, ‘03. Vapour Deposition Hard Facing Physical Vapour Deposition Chemical Vapour Deposition Evaporation Ion Plating Sputtering Welding Thermal Spraying Flame Electric Arc Plasma Arc Cladding Miscellaneous Techniques 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 Deformation Diffusion Brazing Welding Laser Spray & Fuse Low Pressure Plasma Detonation Gun Electric Arc Plasma Arc Flame (HVOF) 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 Selecting coatings • 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 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 C Carbon Diamond 6.71 Å 6.71 Å Graphite DLC Fullerene C60 Fullerene C70 Diamond (sp3) Graphite (sp2) 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-LIKECARBON 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%: 2. Sample Preparation 0.03%C,18%Cr,10%Ni, 3%Mo, Fe (balance) Grinding Thickness: 8mm and 0.25mm, Diameter: 25mm Polishing (240, 600, 1200 grade emery papers Co-Cr and Ti6Al4V alloys (wt%: 69%Co,25%Cr and 5% Mo) and 0.25 diamond polish) Ultrasonic Cleaning Thickness: 8mm and Diameter: 25mm 0.75mm Thickness Glass substrate 3. Saddle field source Neutral beam deposition, energetic molecules Also allows deposition on insulating substrates 4. Argon Etching 5. Film Deposition Parameters In-situ etching - Energetic argon atoms Process Gas: C2H2 and C2H2+Ar gas mixture Time: 0, 05, 10, 15 and 20 min. Voltage: 1-1.7KV Voltage: 1 -1.7KV Current : 0.6A, 1A Pressure: 1.5x10-3 Time: 1 hr. to Current: 0.6, 1A 4.8x10-3 mbar Pressure:1.5x10-3 to 4.8x10-3 mbar Temperature Etching temperature Deposition Temperature NEUTRAL BEAM FAST ATOM SYSTEM Mechanical Characterization ADHESION TESTS F A) Quantitative Adhesion Pull-off Adhesion Stud Epoxy glue Coating Substrate Normal load (1471N) Coated sample B) Qualitative Adhesion Rockwell C Adhesion Rockwell C Adhesion Better Adhesion HF1 or HF2 Medium Adhesion HF3 or HF4 Poor Adhesion HF5 or HF6 Residual Stress in Film Deflection, Bending beam method After deposition Before deposition z-axis STONEY EQUATION: 4Ests2 31 s l 2t f Distance, x-axis (m) = Residual stress Es =Young's modulus of the substrate (200GPa) s = Poisson's ratio of the substrate (0.29) ts = thickness of the substrate tf = thickness of the film l =length of the substrate segment and = largest deflection (usually the central deflection) in the segment measured by surface profilometry after the film deposition with reference to initial deflection. Film Thickness SURFACE PROPHILOMETER