**3.2. Friction coefficient**

juxtaposed lines on the surface of the AISI M2 tool steel. Changes in beam energies and intensities were observed. All conditions used produced metal fusion. They noticed that there was little roughness for the low intensities generated by the laser beam; however, there were "cra-

**Figure 4.** Wear volumes of the untreated and laser-textured surface determined after the ball-cratering wear tests.

**Figure 3.** Wear test conducted on the ISO 5832-1 SS without laser treatment. Ball of AISI 316 L SS.

70 Lubrication - Tribology, Lubricants and Additives

Allsopp and Hutchings [25] have suggested that surface roughness is interesting for improving adhesion between coatings and metallic alloys, and can be produced and controlled by laser beam. This effect is desirable on some biomaterials' surfaces for permanent fixture medical devices. The values of Ra shown in **Table 3** reveal that the average roughness increased

**Specimens Blank 1 2 3 4** Microhardness (HV) 199.3 204.3 215.4 226.1 239.9 Ra (μm) 0.2 1.5 5.2 9.2 11.5

**Table 3.** Microhardness and roughness values for the untreated and laser-textured samples.

ters", that is, regions with high roughness at the higher intensities.

after laser texturing, scaling up with the pulse frequency.

Because wear is a surface phenomenon that occurs at the interface between the asperities of the surfaces in contact, biotribology results are strongly influenced by the surface finish produced by the laser beam texturing.

Considering the ball-cratering wear test, the highest value of the friction coefficient was obtained for the untreated surface as shown in **Figure 5**. The laser-textured specimens presented lower friction coefficient. The lowest value was observed for specimen 3. There was no apparent relationship between the friction coefficient and the laser pulse frequency. Notwithstanding, it is possible to infer that the hardness increase can be related to this effect. Surface roughness, in turn, did not increase the friction coefficient, being the hardness effect more prominent to the friction characteristics of the treated surface. Values of such magnitudes were reported in literature [25–28], with the same type of test under different tribological systems.

In the biomaterials' field for implantable medical or dental devices, tribological assays are of great value in providing an estimate of the normal, tangential, and frictional forces in relation to the volume of material that can be detached from the surface, migration, and accommodation of some particles.

This work also analyzed the evolution of the friction coefficient by nanotribometer wear tests of the surfaces of these biomaterials with laser texturing treatment. The results obtained are presented in **Figure 6**, and are compared with the blank specimen (without treatment).

No direct relationship between wear volume and friction coefficient was observed; i.e., the highest value of wear volume was not related to the higher value of coefficient of friction [25–28].

The variation of the friction coefficient with the test time is shown in **Figure 6** for the laser-textured and untreated samples. These results were obtained by means of the wear tests conducted in the nanotribometer. For the laser-textured surfaces, the values of friction coefficient were lower than those obtained for untreated surface, confirming the results of the ball-cratering wear test.

For the laser-textured surfaces, **Figure 6**, the values of friction coefficient obtained were lower than those obtained in the samples without treatment by the laser beam (blank).

**Figure 5.** Friction coefficient obtained by the ball-cratering wear test for untreated and textured specimens.

**Figure 6.** Variation of friction coefficient as a function of the test time for the laser-textured and -untreated specimens.

The friction coefficient values for the untreated specimen showed a rapid increase in the beginning of the test (running-in), and practically stabilized around values close to μ = 0.5 as the surface becomes less rough. In the case of the laser-textured samples, the friction coefficient decreases up to 100 s, reaching a stabilization period up to the end of the test. For sample 2, the friction coefficient initially presented gradual increase up to 500 s reaching 0.35 and then drops to less than 0.2 at the end of the test. Sample 3 showed a tendency of continuous increment of the friction coefficient with time. For sample 4, in turn, it remained practically constant and at lower than that of the other conditions.

The effect of the variation of the coefficient of friction as a function of the test time was studied by Huang et al. [29]. They verified some tribological properties of Ti-6Al-4 V alloys with and without coating ("laser clad"), for a period of 3500 s in different rotation frequencies, and at the end of the tests, they verified that the coefficient of friction for the coatings was always inferior to the substrate.

is explained by its susceptibility to localized corrosion [5, 6, 32, 33]. According to Pieretti and Costa [5], the laser process affects the corrosion resistance of the laser-treated stainless steel biomaterials, producing a less protective passive film with areas prone to its breakdown,

**Figure 7.** Cell viability as a function of the extract concentration of the laser-textured and untreated samples.

Evaluation of the Biotribological Behavior and Cytotoxicity of Laser-Textured ISO 5832-1…

http://dx.doi.org/10.5772/intechopen.73140

73

**Extracts pH** Control (Pure Ti) 7.50 ISO 5832-1 SS treated by laser 7.88

Studies on the biomaterials tribological behavior are important because they reveal unique aspects about the surface wear mechanisms [34]. The knowledge of wear response contributes to the understanding of other surface phenomena, which can occur simultaneously and

The occurrence of both concomitantly can lead to acceleration of particle detachment, including nonmetallic inclusions that may be housed under biomaterial surface [35], and these may come into contact with the bloodstream and lodge in any part of the human body causing,

although the samples are not considered cytotoxic.

**Table 4.** Values of pH for the extracts at a concentration of 100%.

many times, harm to the patients.

potentiate one another, such as the phenomenon of corrosion.
