Lecture 4 Wear Phenomena D.Sc Harri Eskelinen BK50A2200 Design Methodologies and Applications of Machine Element Design Lecture 4 Wear Phenomena D.Sc Harri Eskelinen

Wear Wear can be divided into SLIDING WEAR, which occurs in the absence of hard particles, and ABRASIVE WEAR, which occurs in their presence. Under some conditions sliding wear can generate debris which then causes further wear by abrasion; it must therefore always be borne in mind that the boundary between different types of wear is not a rigid one. Abrasive wear can be further subdivided into TWO-BODY and THREE-BODY ABRASIVE WEAR. Two-body wear is caused by hard protuberances on the counter face, while in three-body wear hard particles are free to roll and slide between two, perhaps dissimilar, sliding surfaces. A further type of wear involving hard particles is EROSIVE WEAR, where hard particles carried in a gas or liquid stream strike a surface.

Wear In materials science, wear is the erosion of material from a solid surface by the action of another solid. The study of the processes of wear is part of the theory of tribology. There are four principal wear processes: 1. Adhesive wear 2. Abrasive wear 3. Fatigue wear or Surface fatigue 4. Tribochemical wear or Corrosive wear

Also this definition fails to include These four wear phenomena can affect simultaneously but typically one of them is dominant. Wear can be defined as a process in which interaction of surfaces of a solid with the working environment results in the dimensional loss of the solid, with or without loss of material. The definition of wear does not include loss of dimension from plastic deformation, although wear has occurred despite no material removal. Also this definition fails to include - impact wear where there is no sliding motion or - cavitation or corrosion where counter body is a fluid.

Wear environment includes different load types such as - unidirectional sliding, - reciprocating, - rolling, - impact different loading conditions such as - varying speed and temperatures, different counter bodies - solid, - liquid, - gas, and different types of contact such as - single phase or multiphase, in which phases involved can be liquid plus solid particles plus gas bubbles .

Wear along with other aging processes, such as, fatigue, creep and fracture toughness cause progressive degradation of materials with time leading to failure of material at an advanced age. The mechanism of wear is very complex and the theoretical treatment without the use of rather sweeping simplifications is not possible. It should be understood that the real area of contact between two solid surfaces compared with the apparent area of contact is invariably very small, being limited to points of contact between surface asperities. The load applied to the surfaces will be transferred through these points of contact and the localized forces can be very large. The material intrinsic surface properties such as hardness, strength, ductility, work hardening etc. are very important factors for wear resistance, but other factors like surface finish, lubrication, load, speed, corrosion, temperature and properties of the opposing surface etc. are equally important.

Adhesive wear Adhesive wear is also known as scoring, galling, or seizing. It occurs when two solid surfaces slide over one another under pressure. Surface projections, or asperities, are plastically deformed and eventually welded together by the high local pressure. As sliding continues, these bonds are broken, producing cavities on the surface, projections on the second surface, and frequently tiny, abrasive particles, all of which contribute to future wear of surfaces. For adhesive wear to occur it is necessary for the surfaces to be in intimate contact with each other. Surfaces which are held apart by lubricating films, oxide films etc. reduce the tendency for adhesion to occur.

A particle A joint Motion

Excessive load, low speed and/or reductions in fluid viscosity can reduce the oil film thickness to a point where metal-to-metal contact occurs. Surface peeks are "cold welded" together and particles are sheared off as surfaces move.

Abrasive wear When material is removed by contact with hard particles, abrasive wear occurs. The particles either may be present at the surface of a second material or may exist as loose particles between two surfaces. Abrasive wear can be measured as loss of mass by the Taber Abrasion Test according to ISO 9352 or ASTM D 1044. The abrasive wear mechanism is basically the same as machining, grinding, polishing or lapping that we use for shaping materials. Two body abrasive wear occurs when one surface (usually harder than the second) cuts material away from the second, although this mechanism very often changes to three body abrasion as the wear debris then acts as an abrasive between the two surfaces. Abrasives can act as in grinding where the abrasive is fixed relative to one surface or as in lapping where the abrasive tumbles producing a series of indentations as opposed to a scratch.

   

Abrasive Wear Effects: • Dimensional changes • Leakage • Lower efficiency • Generated particles contribute more wear Abrasive wear is a primary wear mechanism. Particles enter the clearance space between two moving surfaces, and act like cutting tools to remove material from the surfaces. The particle sizes causing the most damage are those equal to and slightly larger than the clearance space. To protect opposing surfaces from abrasive wear, particles of approximately the operating clearance size range must be removed.

Estimated hardness values: Pure aluminium 15 HB Pure copper 35 HB Medium carbon steels 120 HB Stainless steels (AISI 304) 250 HB Hardened steels 650…700HB Chromium steels 700 HV Surface plating with Chromium 1000 HB Chromium Carbide 1200 HV Tungsten Carbide 1400 HV Titanium Carbide 2400 HV Diamond 8000 HV

Suitable polymers for wearing conditions PEEK (Polyetheretherketon) PES (Polyethersulfon) PI (Polyimide ) Low friction coefficient High heat resistance (over 150 Celsius) Low wearing rate Possible to reinforce with different fibers

Tribochemical or Corrosive wear A tribological system consists of the surfaces of two components that are in moving contact with one another and their surroundings. The type, progress and extent of wear are determined by the materials and finishes of the components, any intermediate materials, surrounding influences and operating conditions. Tribological system 1 Base object 2 Opponent body 3 Surrounding influences: Temperature, relative humidity, pressure 4 Intermediate material: Oil, grease, water, Particles, contaminants 5 Load 6 Motion

Often referred to simply as “corrosion”, corrosive wear is deterioration of useful properties in a material due to reactions with its environment. One form of high temperature corrosive (oxidative) wear can lead to the formation of compacted oxide layer glazes, which under certain circumstances reduces wear. Erosion Wear due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles Cavitation Erosion A form of erosion causing material to wear by the action of vapour bubbles in a very turbulent liquid. Cavitation is the formation and collapse, within a liquid, of cavities or bubbles that contain vapour or gas. Normally, cavitation originates from changes in pressure in the liquid brought about by turbulent flow or by vibration, but can also occur from changes in temperature (boiling). Cavitation erosion occurs when bubbles or cavities collapse on or very near the eroded surface. The mechanical shock induced by cavitation is similar to that of liquid impingement erosion causing direct localized damage of the surface or by inducing fatigue.

  Tribological sliding contact leads to a chemical reaction. The reaction products influence the tribological processes at the surface; for instance, pairs of components with narrow tolerances can jam. In general, tribochemical wear increases with rising temperature. A frequent cause of tribo-chemical wear is oxidation.

Erosion is caused by a gas or a liquid which may or may not carry entrained solid particles, impinging on a surface. When the angle of impingement is small, the wear produced is closely analogous to abrasion. When the angle of impingement is normal to the surface, material is displaced by plastic flow or is dislodged by brittle failure.

Fatigue wear or Surface fatigue Surface fatigue is a process by which the surface of a material is weakened by cyclic loading, which is one type of general material fatigue. Fretting Wear Fretting is a small amplitude oscillatory motion, usually tangential, between two solid surfaces in contact. Fretting wear occurs when repeated loading and unloading causes cyclic stresses which induce surface or subsurface break-up and loss of material. Vibration is a common cause of fretting wear.

 

Calculation of Wear Rate A common used equation to compute the wear rate is (Archard,1953). Vi =ki × F × s where F is the normal load, s the sliding distance, Vi the wear volume and ki the specific wear rate coefficient. Index i identifies the surface considered.

The k-value is given in m3/Nm or m2/N. From design view the wear displacement h is more convenient than the wear volume V. With hi =Vi /A and the contact pressure p=F/A, where A is the area subjected to wear, we get: hi =ki × p × s The sliding distance s can finally be replaced by s=v × t where v is the mean value for the slide rate and t the running time.

Because the k-value depends just like the friction coefficient on a lot of parameters this factor is to be find experimentally. In standard wear test results, the loss of material during wear is expressed in terms of volume. If wear displacement h is not used, the volume loss gives a truer picture than weight loss particularly when comparing wear resistance properties of materials with large variations in density. For engineering components the working life is over when the dimensional losses exceed the specified tolerance limits.

Advanced equation: V =Ki×SC2 × RC3 V = volume of worn-off material S = surface stress at the contact point R = number of loading cycles Ki , C2,C3 = coefficient depending of material pair and surface properties This advanced equation is developed to take into account also the affects off the peeks in surface profile and affects off repeated loading cycles In some simple load cases also the friction coefficient can be used to estimate the wear rate

TYPES OF LUBRICATION FULL-FILM LUBRICATION • Hydrostatic lubrication - elevated hydrostatic pressure caused with a pump, geometry etc. • Hydrodynamic lubrication (HD) - a small gap between the surface forms the pocket for the lubricant due to right relative speed and size of the gap • Elastohydrodynamic lubrication (EHD) - if the geometry is nonconforming (like in gears ) high speed and enough load is required to keep the lubricant between the surfaces • The thickness of the film and the viscosity are in key-role! MIXED-FILM LUBRICATION • surfaces may occasionally contact with each other though in usual cases lubrication is sufficient BOUNDARY LUBRICATION • surfaces suffer from contacts with each other due to - over loading - lack of lubricant - surface roughness - impurities between the surfaces

Examples Case 1. Affects of wear in a scotch yoke mechanism

Case 1. Affects of wear in a cam-follower mechanism

SUMMARY : WEAR PHENOMENA ABRASIVE WEAR • hard particles destroy the softer surface Theory Application ADHESIVE WEAR • surfaces tend to get stuck to each other Theory Application “scoring or scuffing or galling” DIFFERENT CHAINS OF WEAR PHENOMENA Adhesive Tribochemical Abrasive Adhesive Abrasive Fatique Abrasive Abrasive Tribochemical Adhesive Application TRIBOCHEMICAL WEAR • chemical reactions take place on the surfaces Theory Application “corrosion wear” FATIQUE WEAR • cyclic load affects under the surface layer Theory Application “surface fatigue or pitting”

Exercise 4 Exercise 4A Write brief definitions for the following wear mechanisms (use only one sentence and describe each item with your “own words”) : Adhesive wear Abrasive wear Tribochemical wear Surface fatigue Exercise 4B Try to find different types of applications, in which different wear mechanisms affect as sequential phenomena (eg. adhesive  abrasive). Exercise 4C Select suitable material types for a rolling contact surface, which should withstand abrasive wearing conditions caused by fine sand dust (hardness about 650 HV). Exercise 4D Select suitable surface coatings (or alloying) for a component, which should prevent adhesive wear in case where the construction material is: Steel Aluminium Nickel