Tribological performance of freeze-drying nano-copper particle as additive of paroline oil

The reagents used in the experiment were: Copper(II) sulfate pentahydrate (CuSO₄ ▪5H₂O, AR, purchased from Henan Yunyao Chemical Co., Ltd), Sodium hypophosphite(NaH₂PO₂, AR, purchased from Changzhou Shanglian Chemical Co., Ltd), Ethylenediaminetetraacetic acid (EDTA, AR, purchased from Changzhou Tianheng Industry and Trade Co., Ltd), polyvinyl pyrrolidone (PVP, AR, purchased from Zhengzhou Yuancheng Chemical Co., Ltd), Stearic acid (C₁₈H₃₆O₂, AR, purchased from Jingxian Longyuan Chemical Co., Ltd), ethyl alcohol absolute (C₂H₅OH, AR, purchased from Jinan Yuanxiang Chemical Co., Ltd) and liquid paraffin oil (from PetroChina Company of China).

Different amounts of CuSO₄ · 5H₂O and C₁₀H₁₆N₂O₈ were mixed in a beaker (A), NaHPO₂ and surfactant were placed in a beaker (B) and the two beakers were mixed in distilled water, respectively. The beakers were then placed in a water bath, which was heated to 702 °C. The propeller of the digital single display stirrer was put into the beaker A and mixed through agitator and then the solution in the beaker B was slowly poured into the beaker A for 1 h. After that, the nano-copper solution was washed with distilled water and collected centrifugally. The product was then added to the mixed solution of distilled water and ethanol (volume fraction of 25%), and the seal was stored.

$$\begin{eqnarray}&&2{{\rm{CuSO}}}_{4}+{{\rm{NaH}}}_{2}{{\rm{PO}}}_{2}+2{{\rm{H}}}_{2}{\rm{O}}=2{\rm{Cu}}+{{\rm{NaH}}}_{2}{{\rm{PO}}}_{4}+2{{\rm{H}}}_{2}{{\rm{SO}}}_{4}\end{eqnarray} \tag{ 1 }$$

The above precursor solution was fed into a freeze dryer and kept at −58 °C and freeze-dried at 0.01 mbar for about 24 h. After freeze-drying, the sample was sealed with a pressure capping machine.

In the whole preparation process, the variables of the impacts were too complicated to produce the best preparation process, which contained particle size, the level of influence factors, the number of factors and the number of experiments in the sequence of factors, so the orthogonal experiment was used to optimize the design of the process parameters of the nano-copper particles in the freeze-drying system.

A vacuum freezing-dryer (ALPHA 1–2LD PLUS) was used to dry the particle samples. HX1002T-type electronic balance; Ultrasonic cleaner (KQ2200E) was used to disperse particles to the paroline oil; a transmission electron microscope (TEM, JEM-200CX-type) was used to characterize the granular size. A multi-functional tribometer (UMT-2) was used to test the tribological property of the nano-lubricating oils. The experimental temperature was 20 °C–23 °C. The pressure was 30 N, the sliding frequency was 3 Hz and the reciprocating stroke was 6 mm. On the friction pair: friction pair for Φ = 9.5 mm GGr15 ball, the Φ = 30 mm of the friction pairs were used in the tests. A ball-on-disk reciprocating mod was applied under the lubrication of oil. The sliding time was 3600 s. After the experiments, the friction pairs were ultrasonic cleaning in acetone for 10–15 min A MM-3-type metallographic microscope was used to observe the worn surface.

From the analysis of a large amount of research data, the speed range of the chemical reaction while the preparation of nano-copper particles was found to be 400 ∼ 1000 r/min, so the index of the influence factor A (rotational speed) of this experiment was averaged at 700 r/min and the gradient was 50 r/min. Influence factor B (Types of active agents) was selected as a non-ionic surfactant (PVP) and anionic surfactant (stearic acid). According to the data, the freeze-drying thickness of the material is 5 ∼ 15 mm. So considering that the purpose of this experiment is to explore the particle size of the particles and the study surface, the thickness of the freeze-dried material is thinner, the particle size of the product is smaller. And the particle dispersion is more uniform. The index of influencing factor C (material thickness) was selected to be 5 mm, 7.5 mm and 10 mm, respectively. The freezing rate is a major influence factor of freeze-dried products. Slower freezing rate, uneven frozen material composition and appearance of segregation phenomenon cause the particle size to be difficult to control and it is more likely to obtain a larger particle size of the particles. Therefore, the influence factor D (freezing mode) was selected in three different ways of freezing rate, namely liquid nitrogen freezing (rapid freezing), vacuum freezing (rapid freezing) and −58 ^oC freezing (direct freezing). The results are shown in table 1.

The particle size of nano-copper particles was tested by TEM. The experimental results and particle morphology are shown in table 2 and figure 1, respectively.

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The particle size of the sample prepared by freeze-drying method was uniform, the morphology was similar to that of a spherical body and there was no hard agglomeration between the particles (figure 1). The particle size of the 9th experimental sample was the smallest and was about 3 ∼ 7 nm.

In table 3, K1, K2 and K3 are the sum of the sizes of the freeze-dried nano-copper particle sizes at three levels, k1, k2, and k3 are the average values respectively, and R is extreme difference.

The data in table 3 are calculated by the experimental results (R), the size sequence of influencing factors is R_D (frozen mode) > R_B (active agent type) > R_C(material thickness)> R_A (stir speed), in which the freezing mode is the main influencing factor and the stirring speed before the preparation of the precursor solution has a small influence on the preparation of nano-copper particles. In this experiment, while the preparation of nano-copper particles, particle size affected the goal of the optimal process. In the TEM test, the particle size of nano-copper particles has to be as small as possible. The smaller level of each factor is our goal to optimize for the best effect. From the various levels in factor A, KA3 > KA2 > KA1 was obtained and A1 was selected as the optimal level of factor A. In the same way, B2C1D1 was the optimal level of influence factors of B, C and D. Therefore, through orthogonal experiment, the best freeze-drying production process and the optimal combination of nano-copper particles was A1B2C1D1, i.e. the stirring speed is 650 r/min, the surfactant is PVP, the freeze-dried material thickness is 5 mm and the freezing method is for rapid liquid nitrogen freezing.

table 4 shows the results of orthogonal experiments and analysis of variance. In the table, K_1j, K_2j, and K_3j are the sum of the freeze-dried nano-copper particle sizes at three levels.

The SS_j of each column was calculated according to the following formula.

$$\begin{eqnarray}&&{S}{{S}}_{j}=\displaystyle \frac{1}{r}\,\displaystyle \sum _{i=1}^{m}{{K}_{ij}}^{2}-CT\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{CT}=\displaystyle \frac{{{T}}^{{\rm{2}}}}{{n}}=\displaystyle \frac{{\rm{14641}}}{{\rm{9}}}=\mathrm{1626}{\rm{.778}}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&T=6.5+19+8.5+14+6+22+33+7+5=121\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{SS}_{T}=\displaystyle \sum _{{\rm{i}}={\rm{1}}}^{{\rm{m}}}{{\rm{y}}}_{{\rm{i}}}{\rm{2}}-{\rm{CT}}={\rm{2354.5}}-{\rm{1626.778}}={\rm{727.72}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{S}{{S}}_{{\rm{A}}}=\displaystyle \frac{1}{{\rm{3}}}({{K}_{11}}^{2}+{{K}_{21}}^{2}+K{31}^{2})-CT=\displaystyle \frac{1}{{\rm{3}}}\left(1156+1764+2025\right)-1626.778=21.555\end{eqnarray} \tag{ 6 }$$

Several other values were also calculated using the same method.

$$\begin{eqnarray}&&\begin{array}{l}{S}{{S}}_{{\rm{B}}}=88.722\,{S}{{S}}_{{\rm{C}}}=26.722\,{S}{{S}}_{{\rm{D}}}=590.722\\ \sum \end{array}\end{eqnarray} \tag{ 7 }$$

The degrees of freedom were calculated.

$$\begin{eqnarray}&&d{f}_{A}=d{f}_{B}=d{f}_{C}=d{f}_{D}=3-1=2\end{eqnarray} \tag{ 8 }$$

The variance V_A was calculated using the following formula.

$$\begin{eqnarray}&&{{V}}_{{\rm{A}}}=\displaystyle \frac{{S}{{S}}_{{\rm{A}}}}{{d}{{f}}_{{\rm{A}}}}=\displaystyle \frac{{\rm{21.555}}}{{\rm{2}}}=\mathrm{10}{\rm{.775}}\end{eqnarray} \tag{ 9 }$$

The others were calculated using the same method.

$$\begin{eqnarray*}&&{V}_{B}=44.361\,{V}_{C}=13.361{V}_{D}=295.361\end{eqnarray*}$$

Table 5 shows the variance analysis significance table of the orthogonal experimental results. Combined with table 4, comparing the influence factors of each level in A kij² value, it can be concluded that k_3j² > k_2j² > k_1j², and it can determine the optimal level of A i.e. A₁. Similarly, the optimal levels of the remaining three factors were B₂, C₁, D₁ and the optimal combination of the orthogonal experiment was A₁, B₂, C₁, D₁. The best conditions for the production process of the lyophilized nano-copper particles were as follows: the stirring speed was 650 r/min, the additive was for PVP, and the freeze-drying material thickness was 5 mm and the freezing method was for rapid liquid nitrogen freezing. The variance analysis was carried out with the minimum of A factor as the error term and it can be analyzed from the significance of variance analysis that the difference between D factor and the experimental effect is significant. Finally, the order of influence of the four influencing factors was D > B > C > A, and the data result obtained was consistent with the extreme difference analysis method.

Preparation parameters of nano-copper particles were optimized by orthogonal experiments. The best parameters of preparation for nano-copper particles were: 650 r/min (stirring speed), a surface active agent for PVP, the freeze-dried material thickness of 5 mm and the freezing method of rapid liquid nitrogen freezing. The effect of the freezing method on the particle size of freeze-dried nano-copper particles was larger than that of the stirring speed for the precursor solution.

The same surface modification treatment was performed on commercially available nano-copper particles and freeze-dried nano-copper particles (three nano-copper particles with different freeze methods). A certain amount of stearic acid and Span-80 (4% of the mass of nano-copper particles) was taken in a beaker of 20 ml ethanol and stirred until dissolved. Two kinds of nano-copper particles were washed with dilute nitric acid and dilute sulfuric acid before use. After the drying treatment, they were separately added to the ethanol, containing the surface modifier and then exposed to ultrasonic cleaning for 45 min Two kinds of surface modified copper particles were obtained by drying treatment. The modified nano-copper particles (0.5% by mass of paraffin oil) were added to the paraffin oil separately, exposed to ultrasonic treatment for 1 h, and then collected in a test tube. After collection, they were left standstill and then observed. The stability performance of different oil samples was evaluated after sedimentation and stratification.

Figure 2(a) shows the result of the oil sample after vacuum freezing of nano-copper particles standing still for 18 h. The sample is black. Figure 2(b) shows the result after 9 days of standing still. Figure 2(c) is the result of the commercially available nano-copper particle oil sample after standing still for 10 h and the oil sample is reddish brown.

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After 18 h, the vacuum-frozen nano-copper lubricating oil was found to have good dispersion stability and no phenomenon of sedimentation or delamination was observed (figure 2(a)). After standing still for 9 days (figure 2(b)), the oil sample showed a settlement of about 10%, indicating better stability of the particles. The −58 °C frozen and liquid nitrogen frozen nano-copper particles had different degrees of delamination after standing in the lubricating oil for 1 day and they all settled after 3 days. Compared with vacuum frozen nano-copper particles, these two samples had weaker stability when added to paraffin oil. However, the commercially available nano-copper particles lubricant samples showed about 20% of oil sample delamination after 10 h and the sedimentation of nano-copper particles in the lubricating oil was more serious, indicating its dispersibility to be unstable. After simple and identical surface modification treatment, the dispersion stability of freeze-dried nano-particles was found to be better than that of common copper particles in paraffin oil and the effect of vacuum frozen nano-particles was found to be the best. In addition, the pH values for all the lubricating oil samples were about 7.0.

More studies have been done about that that the tribological properties of nanoparticles as lubricants additives depend on particle size, minor scale size of nanoparticles have better lubrication property [28, 29]. So only the small-size product for the best preparation technology was used to test the tribological property. Based on the above freeze-dried nano-copper particles samples as additives and liquid paraffin oil as the base oil, different concentrations of paraffin oil with nano-copper particles additives (experiment) were prepared with the concentration (mass ratio) : 0.0%, 0.02%, 0.05%, 0.1% and 0.3%, respectively.

Figure 3 shows the COF curve of paraffin oil at various concentrations of additives. It can be seen from the figure that the friction coefficient increases with the increase of friction reaction time and the friction coefficient increases from 0.12 of 600 s to 0.135 of 3600 s. However, the friction coefficient of the paraffin oil added to the nano-copper particles was found to be relatively stable and the friction coefficient of the paraffin oil with relatively no nano-copper was less volatile, indicating that the freeze-dried nano-copper particles have a good lubrication effect. Different concentrations of nano-copper particles in paraffin oil had different effects. When the concentration of copper particles was 0.05 ∼ 0.1 wt%, the oil sample obtained the lowest COF.

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When the additive amount was 0.3 wt%, the range of COF increased, indicating that excessive additive amount cannot improve the tribological property of the lubricating oil. Particles collided with each other, easily produced larger particles, increased the particle size of copper particles in the lubricating oil, at 30 min, the friction coefficient and friction properties of the liquid paraffin oil were flat, then continued to increase, and verified with the increase of reaction time, impurities in lubricating oil caused the gradual reduction of friction performance of paraffin oil.

Figure 4 shows the wear scar maps and wear rates of the tested ball. The wear scar radius of 0.05 wt% nano-copper additive obtained the lowest wear rate. When the copper additive was lower than 0.05 wt% or exceeded 0.05 wt%, the wear rates of the lubricating oils increased. Besides, all the oil samples containing copper particles showed a similar abrasion mechanism of micro-plowing. As the copper particles deposited on the surface during the sliding process, copper particles played the role of self-healing to some extent. Figure 5 shows the worn images for the lower disks. Figure 5(a) shows that worn surface presents serious ploughing wear and pits for the non-additive paraffin oil sample. With the addition of copper particles into paroline oil, the anti-wear property can be improved.

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When the concentration of copper particles reached 0.05 wt%, the wear-resisting performance was found to be the best. When the concentration of copper particles was 0.05 wt%, only some slight scratches appeared on the worn track. Although the damage situations still existed for the samples of 0.05 wt% and 0.1 wt%, the worn degrees were improved than that of containing no additive or high concentration of oil samples. As shown in figure 6, the potential reason is that the appropriate concentrations of copper particles just fill the surface pits, which is beneficial for the improvement of the tribological performance and the excessive copper particles overflow the pits, resulting in the aggravation of surface wear [30, 31]. Besides, the viscosity of the composite lubricating oil contained nano-copper particles increased than that of pure paroline oil, this factor can improve the elastohydrodynamic lubrication effect [32].

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