Fundamentals of Tribochemical wear_pptx

1. FUNDAMENTALS OF TRIBOCHEMICAL WEAR
TRIBOLOGY OF MATERIALS (CR-4102)
ASSIGNMENT
Submitted By:- Course Mentor:-
Sanket Kumar Dani Prof. Debasish Sarkar
118CR0673
Department of Ceramic Engineering
National Institute of Technology, Rourkela, 769008

2. INTRODUCTION:-
WEAR:-
 It is defined as the continuous removal of materials from contacting surfaces in relative motion.
 It actually results due to friction and occurs at the zone of contact.
 The process of this wear can occur in a wide range of substances i.e metals ,non-oxides,ceramics and polymer.
Basically ,wear can be of three types based on the intensity of its damage and quality of material lost by an active
element;
• Mild wear:- It is one of the basic types of wear which has less loss or damage or we can say impact on the
materials.In this type of wear mechanism ,it involves the reaction with environment as a result of which there is
formation of oxides in the wear. In this the system is worn away very little , surface remains smooth and particles
have dimensions in order of micrometer.
• Severe wear :-In this type of wear the intensity of damage of materials is much more than normal wear ,which can
be calculated finally from the wear rate value. Also in this the body surfaces are changed and the size of particle
even exceed 100 micrometer.
• Catastrophic wear :- In this type of wear particularly life expectancy of the particle is reduced exponentially.

3. Wear rate :-
o It can be defined as the volume worn per unit load per unit traversed distance from contacting surfaces in relative
motion.
o Its unit is m3/Nm .
wear rate= (volume loss * hardness) / (sliding distance * applied load).
Factors that influences the level of wear are as follows :-
(i) Force of contact
(ii) Temperature
(iii) Area of contact between the contacting surfaces
(iv) Physio – Chemical state of surfaces in friction
(v) Crystallographic structure and material properties of materials ,one of them being the hardness.
Wear can proceed in several way in the materials ;that can be through plastic deformation in which there is creation ,
displacement of dislocations,impacts ,scratches will occur Or it can take the form of fracture as well in brittle materials
by intiation,accommodation,progression and propagation.

4. Origin of wear :-
Archard’s Experimental Law :-
According to Archard experimental law , the wear volume is proportional to the normal load , the sliding distance and
inversely proportional to the hardness of the contact materials.
Archard wear equation which is used to gain insight on wear is based on the theory of asperity contact.
where, Q or It can also be denoted as (Wv) = Total volume of wear produced
k= wear coefficient
W = Normal force or load
L = sliding distance or distance traversed
H = Hardness of the contacting surface

5. Types of wear mechanism :-
(i) Adhesive wear
(ii) Abrasive wear
(iii) Oxidative wear
(iv) Fretting wear
(v) Erosive wear
(vi) Tribochemical wear
-> For any technological applications , not single but combination of wear mechanisms are used.
-> Of the above mentioned wear mechanisms, one or more even maybe operating in one particular piece
of machinery. In many cases even these type of scenario arises where the wear is initiated by one
mechanism and it then proceed to other wear mechanisms which ultimately leads to complicacy in failure
analysis of wear.

6. TRIBOCHEMICAL WEAR :-
 Frictional heating modifies the kinetics of chemical reaction.
 Tribochemical wear is one of the most complex type of wear.
 Tribochemistry can be defined as the chemistry that deals with the chemistry of chemical reactions by friction that
actually occur between the lubricant and the surface under boundary condition.
 And the wear governed by these kinda reactions is called tribochemical wear.
 Due to the result of mostly the friction in the chemical reactions , the extent of reactions of sliding bodies increased
as a resut of which reactions occurs at a higher temperature than normal temperature which is the main reason of
arise of this type of wear.
 The most common method or mechanism by which friction increases the rate of chemical reactions is by producing
heat at contacting surface of asperities.
 Tribochemical wear also results in oxidative wear of ceramics,metals and development of friction polymer films.
 Tribochemical reaction is due to the chemical reactions but initiation may take place due to adhesive or abrasive
wear or other mechanical wear.

7.  The interface temperature produced at the asperity contact during sliding of two counterbodies which is known as flash
temperature.
 This temperature is basically responsible for thermal oxidation and produce oxide fims in the range of nanometer to several
micrometer.
 At low temperature oxidation occurs at asperity contacts from frictional heating
Here the low temperature means the environmental temperature not the flash temperature.
 Whereas,at high temperature generally the oxidation of entire surface occurs and affects wear .
Example – of tribochemical
wear of ceramic
cutting tool.
 (a) Schematic drawing showing a tribologically induced transfer layer on the contact surface of the ceramic sample,
(b) SEM image of a cross section showing Tribochemical wear products on the ceramic contact tip.

8.  A Tribological system basically consists of surface of two components that are in contact with one another or each other
and surrounding while moving .So, the progress, type and the extent of wear are determined by the materials and finishes
of the components ,any intermediate materials , influence of surrounding and the operating conditions.
 Tribological System :-
(i) Base body
(ii) Opponent body
(iii)Influence of surrounding such as ;
Temp. , Relative humidity , pressure
(iv) Intermediate materials ; Oil , grease
water , contaminants , particles
(v) Load
(vi) Motion

9.  In tribochemical wear mostly there is synergetic effect of chemistry as well as the mechanical wear ,that’s how it got its name
of tribochemical wear.
 These chemical reactions in these wear are often not expected or feasible under the same temperature and static load
conditions;
 Tribochemical reactions are also different from normally observed thermochemical reaction with respect to their kinetics as
this wear normally results from the removal of reaction products formed in situ from contacting surfaces.
Tribochemical wear Examples :-
• Tight fitting surfaces are particularly endangered with this type of wear.
• Tribochemical wear mostly observed in;
(i) Between race and shaft/ Housing and roller bearing
(ii) Bevel-wheels
(iii) Rivoted joints and clutches
(iv) Links of chain
(v) Plate springs etc.

10. Mechanisms of Tribochemical wear:-
The products that are formed during the chemical reactions of tribochemical wear are resulted mostly from the reaction of
solid surfaces with enivironment or between the mating materials.
 If we take the instance of a metallic material system, the metallic contact of the surface asperities leads to removal of metal
due to adhesion.
 Then the protective surface layer that is formed due to chemical reaction of metals with the environment will result in
reduction of metallic contact.
 Then due to excess locally high pressure ,there is cracking of protective surface layer.
 Metalic and non metallic wear debris may act as abrasive and roughen the contacting surface.Then finally formation of
protective layer may lead to smoothening of surfaces again.

11.  Hong and its Co- workers developed a model for quantifying the wear loss due to tribochemical reactions (oxidation) .
where,
V or Wv = wear volume
A = Area of contact
Ai = Arrhenius Constant
Q = Activation energy
Rg = Molar Gas constant
Tf = Flash Temperature
p0 = Average density of the oxide in contact
fo = Mass fraction of the oxide
Ts = Sliding Time

12.  Not always these tribochemical reactions are having negative impact on wear; there are also several times when these
tribochemical reactions are having positive impact on wear and those are mostly associated and designed for surface
protection and friction reduction.
 These tribochemical layers also act as load bearing surfaces which posses low shearing strength, that results in low friction
and low wear.
Prevention from Tribochemical wear :-
o These tribochemical wear cannot be completely prevented but by effective process it can be reduced to certain extent and
for this lubricating and coating the surfaces are among the best methods.
o Generally the conventional lubricants that are essentially made for steel surfaces are mostly used for lubricating machine
surfaces or component surfaces having protective coating such as DLC’s( Diamond like carbon) .DLC’s comes under the
hard coatings which are still extensively used due to their relatively low friction and superior wear resistance compared to
other hard coatings. Oil and Grease are most commonly used for the purpose which helps in keeping the surfaces apart
from each other that helps in prevention of these wear.
o Another one of the conventional method of this wear reduction is by simply changing the material properties eg. By
hardening as we know that wear volume is inversely proportional to hardness.

13. THANK YOU !