Dry sliding wear performance of an annealed TiNi alloy with different nickel contents

To gain a deep understanding of the effects of the Ni content on the mechanical properties of the processed alloys, indentation tests were conducted using a Nanoindenter XP with a Berkovich diamond pyramid indenter at ambient temperature. The Olivier and Pharr analysis method [16] is used to assess both the hardness and Young's modulus of the alloys. The load used is 50 mN. The determination of these two performed under a load of 50 mN required an average of 20 tests for each material.

The wear tests were conducted without lubrication at the ambient temperature in accordance to the ASTM G99-95a. A pin-on-disc tribometer (CSM Instruments) with a counter bodies as a alumina ball with 6 mm-diameter for a total sliding distance of 20 m. The friction coefficient was recorded automatically during the test thanks to the Tribox 4.49 data acquisition software. To predict the coefficient of friction (COF), the friction tests were conducted according to the factorial planning model 2² according to the combinations shown in table 1.

Table 1.  Coded factors for the tribological tests.

		Levels
Coded factors	Imput paraeters	−1	+1
${X}_{1}$	Load Py (N)	2	4
${X}_{2}$	Nickel content (%)	40	60

The mathematical model used is a polynomial model of the first degree [17] with the interactions expressed as below equation (1)

$$\begin{eqnarray}&&Y={a}_{0}+{a}_{1}X{}_{1}\,+\,{a}_{2}{X}_{2}+{a}_{12}{X}_{1}{X}_{2}\end{eqnarray} \tag{ 1 }$$

Where:

${a}_{0}:$ constant coefficient of model;

${a}_{1}:$ coefficient of factor 1;

${a}_{2}:$ coefficient of factor 2;

${a}_{12}:$ coefficient of term $X{}_{1}X_{2};$

$Y:$ Response.

The wear volume was estimated using 3D profilometry image (laser source profilometer, Cyber Technology CT100), by measuring the surface profile of the wear track. The morphology of the worn surfaces, were analyzed by means of a scanning electron microscopy (Quanta250-FEI).

In addition, the indentation and the tribological behaviours of the TiNi alloys were compared with other conventional materials, TA6V4 alloy tested at room temperature and at identical processing conditions.

Figure 2 displays the variation of the friction of coefficient (COF) ('f') for the TiNi and TA6V4 alloys as a function of the sliding distance under different applied loads of 2 and 4 N.

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These curves indicate two successive friction period. The first is the running in period during which the coefficient increases rapidly to reach a maximum value. This period is characterized by an important wear and plastic deformation of surface asperities. The second period corresponds to the stabilization of the friction coefficient whose value is maintained constant regardless of sliding distance. This step corresponds to the formation of solid wear debris (third body) during the sliding process [8, 20, 21]. The TA6V4 alloy generated a low value of the friction coefficient ('f') (table 3) compared to the TiNi alloys for both loads. This can be attributed to the formation of two TiO₂ and Al₂O₃ oxides that tend to form on the worn surface of the alloy and will thereby effectively reduce the coefficient of friction [5]. The average friction coefficient values are shown in table 3.

The experimental results of wear tests have resulted in a relation between the studied response 'f' (Y) and the input parameters which are the applied load 'Py' (X1) and the Nickel percentage 'PNi'( X2), thus enabling, to predict the friction coefficient 'f' (Y) in study field. The prediction model of friction coefficient 'f' can be written in function coded factors (X1 and X2) (equation (3)) or in legal values (equation (4)).

$$\begin{eqnarray}&&f=0.65-0.01X{}_{1}-0.015{X}_{2}+0.005{X}_{1}{X}_{2}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&f=0.83-0.035Py-0.003PNi+0.0005Py.PNi\end{eqnarray} \tag{ 4 }$$

The effect of the friction parameters (load 'Py' and Nickel % 'PNi') on the friction coefficient 'f' can be evaluated using the iso-responses curves (figure 3(a)). It appears that when the percentage of Nickel is at its maximum level and whatever the normal load applied, the friction coefficient decreases to a value of 0.63 for Py = 4 N. This can be explained by the fact that the increase in the load causes the formation of a large amount of wear debris, which accumulates in the contact zone and minimizes friction of two rubbing bodies. On the other hand, for the low levels of Py (X1) and PNi (X2) (figures 3(b) and (c)), the friction coefficient increases to reach the value of 0.68. This may be due to the low volume fraction of the intermetallic formed.

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The wear rate was obtained by means of Archard formula [22]

$$\begin{eqnarray*}&&{W}_{s}=V/P\cdot L\end{eqnarray*}$$

Where:

${W}_{s}$ is the wear rate (mm³/N/m),

$V$ is the wear volume (mm³),

$P$ is the applied load (N),

$L$ is the sliding distance (m)).

The values of wear rates obtained from the wear tests carried out for different alloys are reported in table 4. The experimental results seem to be confirmed by Archard's law according to which it is assumed that the volume of material lost is inversely proportional to the hardness value of the material [23]. Therefore, TiNi alloys do not show a significant weight loss compared to the TA6V4 alloy, which has a visibly higher wear rate value.

The effect of the independent parameters on the specific wear rate is illustrated by the three-dimensional simulation (3D) (figure 4). The results show that the wear rate increases with the increase in applied load. This results first in the increase of load, which causes an increase of pressure on the contact surface. Secondly, the high load applied may cause the breakdown of the protective oxide film formed on the surface of the sample due to the temperature rise induced by friction, while the increase in Ni content results in a decrease of the wear rate. This suggests that the amount of nickel in the alloys tested played an important role in their friction behavior.

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The TiNi60 alloy exhibited a low wear rate, due to its better superelasticity. The reason for improving the wear resistance of TiNi alloys over TA6V4 can be attributed to its lower E/H and higher recovery ratio (table 2). Recent studies have shown that wear depends strongly on the value of E/H, contrary to the traditional idea that considers hardness as the principal parameter controlling the wear mechanism [21]. The difference between the wear rate and the surface wear characteristics of the different alloys can be partly attributed to the difference in hardness and deformation process associated with the microstructural characteristics.

The wear tracks for the TiNi and TA6V4 alloys under load of 2 N and 4 N are shown in figures 5 and 6, respectively. Their wear morphology of the tested specimen at 2 N load depicts several grooves with different depths and widths. In addition, for the loading condition of 4 N, it is found an increase in the depth of wear grooves which can be a sign of the increase in the wear rate.

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The SEM micrographs of the worn surface for the different TiNi and TA6V4, respectively, under charges of 2 N and 4 N (figures 7 and 8), reveal wear debris resulting primarily from the destruction of the surface asperities, as well as the degradation of hardened oxidized layers. These phenomena inevitably produce a third body involved in the process of surface degradation [20]. We note the presence of grooves parallel to the sliding direction, confirming abrasive wear mechanism. The latter is induced by hard phases (intermetallic and oxides) and by wear debris of deformed and plastically hardened parts of the alloy [5]. There is also a sign of delamination at the contact surface suggesting the presence of adhesive wear.

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