**6. Characteristic mechanisms in the superficial layers of contacts implying polymeric materials**

Neale admitted that wear is a complicated process and even if the mechanisms could be described, there are combinations and transitions among them that make them difficult to be understood yet and reduced [52]. Four main wear mechanisms are discussed in literature [23, 46]: abrasion, adhesion, fatigue and tribo-corrosion, with particular, mixt variants (thermal and tribofatigue, fretting etc.).

Aspects of wear mechanisms with different adding materials in polymers are well described and interpreted in [3, 8, 20, 46]. A particular wear process of polymeric materials is the so-called delamination, that is a combined process of sublayer crack, plastic deformation and material removal (**Figure 32**).

Forms of abrasive wear are micro-cutting, plowing and micro-cracking with material remove are particularized for polymers that are visco-plastic materials.

Adhesion has particular aspects in tribosystems with polymers, including

polymeric transfer on the counter surface, especially when this is made of steel. As Stachowiak and Batchelor [46] mentioned, this transfer has two extreme consequences:

• beneficial, when the transfer film is thin and transform the moving contact in polymer-polymer,

The solution of reducing wear of polymers is to add materials that keep the polymer into a network (random or organized) to minimize the polymer volume implied in the local deformation and detaching small wear particles instead of big ones. The research has to establish an optimum concentration of constituents that allow for having a better tribological behavior (reduced wear, permissible working temperature, low power loss due to friction and to keep the functions of the systems

*Typical aspects of the failure mechanisms in sliding on steel in dry regime (a) adhesive wear, (b) abrasive wear,*

*SEM images for tribolayers: PA disk (a) and for the composite with 50% glass beads (b, c and d), dry sliding on steel (no gold coating of the samples) for SEM investigation. (a) v = 0.5 m/s, p = 1 MPa. (b) v = 0.5 m/s,*

*p = 2 MPa. (c) v = 1 m/s, p = 1 MPa. (d) v = 1,5 m/s, p = 1 MPa [11].*

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

For instance, Maftei [11] elaborated composites with glass beads in a polyamide matrix with concentration between 5% wt and 50% wt and tested them on pin-ondisk tribotester. SEM investigation revealed agglomerated glass beads, a very thin soften layer of polymer that cover like a blanket the glass beads, justifying the still low friction coefficient. The next figures (**Figure 33** and **Figure 34**) point out differences between wear mechanisms for PA6 (a) (abrasive, fatigue with small cracks) and the composite (detaching smaller polymer debris, al lower sliding velocity the soften layer does not exists and polymer is deformed by the random small movements of the beads in the matrix, at higher velocity (d) several beads roll

Typical aspects of the failure mechanisms in polymer sliding against harder bodies are described in [53–55]: abrasive wear, adhesion wear (with transfer) and

The geometry of the reinforcement makes the wear mechanism to be different for the same fibers, if the matrix is different, as one may see in **Figure 36**. The first line of SEM images is for the matrix of PA6, more ductile than PBT - the matrix of the composite in the second line of SEM images. All tests are done on block-on-ring tester, in dry regime. A more ductile matrix is easier worn and torn-off, the fibers remaining to bear the load and there visible the deformations (flows) induced by a higher load on the fiber ends. In a PBT matrix, more rigid than PA6, the transfer on

the steel counterbody is less and the fibers are scratched under higher load.

in an reliable range).

**Figure 34.**

**Figure 35.**

*(c) fatigue wear [11].*

fatigue wear (**Figure 35**).

**91**

in the superficial layer as the polymer is less viscos.

• not beneficial, with lump or insular transfer, that change too much the surface topography.

**Figure 32.**

*Wear deterioration of a polymeric body in sliding against a harder material, also known as delamination [35].*

**Figure 33.**

*SEM images on tribolayer generated from composites with PA6 matrix and different concentrations of glass beads [11].*

*Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

**Figure 34.**

concentration of PEEK, the wear is dominated by fatigue cracks and the microreservoirs of PTFE are in reduced number and the solid lubrication of PTFE is done only on patches. Wear debris made of PEEK generate a more intense abrasive wear, even as third body, care damage the transfer films on both surfaces in contact. A similar tribological behavior was noticed by Tomescu [9], when a composite

**6. Characteristic mechanisms in the superficial layers of contacts**

with particular, mixt variants (thermal and tribofatigue, fretting etc.).

layer crack, plastic deformation and material removal (**Figure 32**).

Neale admitted that wear is a complicated process and even if the mechanisms could be described, there are combinations and transitions among them that make them difficult to be understood yet and reduced [52]. Four main wear mechanisms are discussed in literature [23, 46]: abrasion, adhesion, fatigue and tribo-corrosion,

Aspects of wear mechanisms with different adding materials in polymers are well described and interpreted in [3, 8, 20, 46]. A particular wear process of polymeric materials is the so-called delamination, that is a combined process of sub-

Forms of abrasive wear are micro-cutting, plowing and micro-cracking with material remove are particularized for polymers that are visco-plastic materials. Adhesion has particular aspects in tribosystems with polymers, including polymeric transfer on the counter surface, especially when this is made of steel. As Stachowiak and Batchelor [46] mentioned, this transfer has two extreme

• beneficial, when the transfer film is thin and transform the moving contact in

• not beneficial, with lump or insular transfer, that change too much the surface

*Wear deterioration of a polymeric body in sliding against a harder material, also known as delamination [35].*

*SEM images on tribolayer generated from composites with PA6 matrix and different concentrations of glass*

copper + PTFE was tested in dry and water lubrication regime.

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

**implying polymeric materials**

consequences:

**Figure 32.**

**Figure 33.**

*beads [11].*

**90**

polymer-polymer,

topography.

*SEM images for tribolayers: PA disk (a) and for the composite with 50% glass beads (b, c and d), dry sliding on steel (no gold coating of the samples) for SEM investigation. (a) v = 0.5 m/s, p = 1 MPa. (b) v = 0.5 m/s, p = 2 MPa. (c) v = 1 m/s, p = 1 MPa. (d) v = 1,5 m/s, p = 1 MPa [11].*

**Figure 35.**

*Typical aspects of the failure mechanisms in sliding on steel in dry regime (a) adhesive wear, (b) abrasive wear, (c) fatigue wear [11].*

The solution of reducing wear of polymers is to add materials that keep the polymer into a network (random or organized) to minimize the polymer volume implied in the local deformation and detaching small wear particles instead of big ones.

The research has to establish an optimum concentration of constituents that allow for having a better tribological behavior (reduced wear, permissible working temperature, low power loss due to friction and to keep the functions of the systems in an reliable range).

For instance, Maftei [11] elaborated composites with glass beads in a polyamide matrix with concentration between 5% wt and 50% wt and tested them on pin-ondisk tribotester. SEM investigation revealed agglomerated glass beads, a very thin soften layer of polymer that cover like a blanket the glass beads, justifying the still low friction coefficient. The next figures (**Figure 33** and **Figure 34**) point out differences between wear mechanisms for PA6 (a) (abrasive, fatigue with small cracks) and the composite (detaching smaller polymer debris, al lower sliding velocity the soften layer does not exists and polymer is deformed by the random small movements of the beads in the matrix, at higher velocity (d) several beads roll in the superficial layer as the polymer is less viscos.

Typical aspects of the failure mechanisms in polymer sliding against harder bodies are described in [53–55]: abrasive wear, adhesion wear (with transfer) and fatigue wear (**Figure 35**).

The geometry of the reinforcement makes the wear mechanism to be different for the same fibers, if the matrix is different, as one may see in **Figure 36**. The first line of SEM images is for the matrix of PA6, more ductile than PBT - the matrix of the composite in the second line of SEM images. All tests are done on block-on-ring tester, in dry regime. A more ductile matrix is easier worn and torn-off, the fibers remaining to bear the load and there visible the deformations (flows) induced by a higher load on the fiber ends. In a PBT matrix, more rigid than PA6, the transfer on the steel counterbody is less and the fibers are scratched under higher load.

the wear rate at 180°C. Wear rates increase at high loads, but brittleness is not obvious till 150 N, at high temperatures. A discontinuous platelet transfer film

*Images of the partial bearings made of PTFE + short glass fibers with different concentrations, test conditions: v = 2.5 m/s and p = 4.6 MPa, water lubrication, Lx = 10,500 m [8]. (a) 15% glass fibers. (b) 25% glass fibers.*

Thermoplastic polyimides show three sliding regimes that are related to a

• at 100 to 120°C, friction increases and is higher and wear rates are lower as compared to sintered polyimides; a thin transfer film develops; dark wear

• at 120 to 180°C, friction decreases and a transition to high wear rates is

• at 180–260°C, friction increases and overload wear results from melting; a thick transfer film develops, and the polymer surface smoothens. Roll-like debris are visually observed as an indication for melting. Raman investigation indicates thermal decomposition of aromatic structures into amide monomers on the polyimide surface, weakening strength and producing higher wear.

And study point out the importance of test parameters, here the two polymers,

Agglomeration of reinforcement fibers of particles are observed even in lubricated system with polymer composites sliding against steel. A suggestive model of reinforcements agglomeration in the superficial layer of polymeric composites, due to preferential wear of the polymer matrix has been described by Blanchet and Kennedy [10] from 1992, and then developed by Han and Blanchet in 1997 [57] and experimental results given in **Figure 37** sustained their model. Each worn surface after sliding in water has a similar concentration in short glass fibers, even if

the temperature and the load. Such a study could be done for each polymer of interest, with particular values for the test parameters, as they do not have a pattern

due to their diversity in chemical structures and molecular organization.

**7. Tendency in using polymeric materials and conclusions**

New development in processing polymer-based materials (here including polymers, polymer blends, polymer composites and stratified materials based on polymeric fabrics) make easier to replace metallic parts with ones made of

initiated; a patchy-like transfer film develops and the polymer surface becomes irregular and opaque due to softening and chemical modification; wear debris

develops above 180°C.

**Figure 37.**

*(c) 40% glass fibers.*

combination of chemical and thermal effects.

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

particles were produced by hydrolysis,

become brittle and act as an abrasive,

initially the concentrations were different.

polymer-based materials, at a convenient price.

**93**

**Figure 36.** *Block-on-ring, L = 5000 m (thin gold coating of the samples) [16].*

Composites with reinforcing particles or fibers: dynamic wear process, in stages: 1 - low wear of polymer and enrichment of the superficial layer in harder materials, 2 - too much hard particles or fibers within tribolayer, the result being big wear particles torn up in bigger conglomerate, 3 - leveling the rough surface after detaching hard particles/fibers by the help of plastic matrix (friction coefficient has high oscillations and the process is repeating.

Friction materials, as for brake pads, need special attention as they have to fulfill requirements as constant friction coefficient and controllable wear (linear would be better). Manoharan et al. [55] presented a study for a composite containing nine major ingredients, including epoxy resin, reinforcement, solid and liquid lubricants etc. (this pointing out the complexity of a composite destinated for brakes). Tests done on disk-on-plate tribotester, in the presence of third body (sand), revealed that wear volume loss of composite brake pad increases with increasing sliding distance and load, but wear rate increases with applied load and decreases with increasing sliding distance. Glass fibers and hard particle fillers were effective in reducing wear rate of the composite. It is reasonable to deduce that binders would increase the adhesion of glass fibers, SiC into the formaldehyde matrix. When the load is increased, microcracks are formed, followed by fragmentation in composite brake pad. Plowing, cracking and accelerated breakage of fibers in composite are evident under higher load. This study is here given in order to underline the necessity of testing new formulated friction materials, no theoretical model being able to reliably predict the tribological behavior in terms of values for wear, friction and durability.

Samyn et al. [56] presented a useful review on tribology of polyimides. Temperature modifies the tribological behavior of this polymer by chemical effects.

The tested sintered polyimides show two sliding regimes: between 100°C and 180°C, friction is high and wear rate increases, with a discontinuous minimum at 140°C. Raman spectroscopy motivated that hydration generates a reversion of polyimide into a precursor. A maximum hydrolysis intensity at 140°C explains the minimum wear rate with acid groups acting as a lubricant. From 180–260°C, friction decreases and wear rate become stable at mild loads, with a maximum value for *Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

**Figure 37.**

Composites with reinforcing particles or fibers: dynamic wear process, in stages: 1 - low wear of polymer and enrichment of the superficial layer in harder materials, 2 - too much hard particles or fibers within tribolayer, the result being big wear particles torn up in bigger conglomerate, 3 - leveling the rough surface after detaching hard particles/fibers by the help of plastic matrix (friction coefficient has

Friction materials, as for brake pads, need special attention as they have to fulfill requirements as constant friction coefficient and controllable wear (linear would be better). Manoharan et al. [55] presented a study for a composite containing nine major ingredients, including epoxy resin, reinforcement, solid and liquid lubricants etc. (this pointing out the complexity of a composite destinated for brakes). Tests done on disk-on-plate tribotester, in the presence of third body (sand), revealed that wear volume loss of composite brake pad increases with increasing sliding distance and load, but wear rate increases with applied load and decreases with increasing sliding distance. Glass fibers and hard particle fillers were effective in reducing wear rate of the composite. It is reasonable to deduce that binders would increase the adhesion of glass fibers, SiC into the formaldehyde matrix. When the load is increased, microcracks are formed, followed by fragmentation in composite brake pad. Plowing, cracking and accelerated breakage of fibers in composite are evident under higher load. This study is here given in order to underline the necessity of testing new formulated friction materials, no theoretical model being able to reliably predict the tribological behavior in terms of values for wear, friction and durability. Samyn et al. [56] presented a useful review on tribology of polyimides. Temper-

ature modifies the tribological behavior of this polymer by chemical effects.

The tested sintered polyimides show two sliding regimes: between 100°C and 180°C, friction is high and wear rate increases, with a discontinuous minimum at 140°C. Raman spectroscopy motivated that hydration generates a reversion of polyimide into a precursor. A maximum hydrolysis intensity at 140°C explains the minimum wear rate with acid groups acting as a lubricant. From 180–260°C, friction decreases and wear rate become stable at mild loads, with a maximum value for

high oscillations and the process is repeating.

*Block-on-ring, L = 5000 m (thin gold coating of the samples) [16].*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

**Figure 36.**

**92**

*Images of the partial bearings made of PTFE + short glass fibers with different concentrations, test conditions: v = 2.5 m/s and p = 4.6 MPa, water lubrication, Lx = 10,500 m [8]. (a) 15% glass fibers. (b) 25% glass fibers. (c) 40% glass fibers.*

the wear rate at 180°C. Wear rates increase at high loads, but brittleness is not obvious till 150 N, at high temperatures. A discontinuous platelet transfer film develops above 180°C.

Thermoplastic polyimides show three sliding regimes that are related to a combination of chemical and thermal effects.


And study point out the importance of test parameters, here the two polymers, the temperature and the load. Such a study could be done for each polymer of interest, with particular values for the test parameters, as they do not have a pattern due to their diversity in chemical structures and molecular organization.

Agglomeration of reinforcement fibers of particles are observed even in lubricated system with polymer composites sliding against steel. A suggestive model of reinforcements agglomeration in the superficial layer of polymeric composites, due to preferential wear of the polymer matrix has been described by Blanchet and Kennedy [10] from 1992, and then developed by Han and Blanchet in 1997 [57] and experimental results given in **Figure 37** sustained their model. Each worn surface after sliding in water has a similar concentration in short glass fibers, even if initially the concentrations were different.
