Basalt fibers used in this study were supplied by Mukhtha Giri industrial corporation, Mumbai in the form of Continuous Basalt Fibers (CBF). These are natural fibers produce by crushing the volcanic rocks and then melting them between temperatures 1300˚C - 1700˚C to produced into fine fibers. They have average diameter of 6 μm, an elastic modulus of 90 GPa, and a yield stress of 4500 MPa. The continuous fibers were cut down into short fiber to length about 0.5 mm, although the mechanical method used gives wide length distribution between 50 and 700 μm. The chemical composition of basalt fiber is shown in Table 1.

The cut short basalt fibers were coated with copper using electroless method. The process of coating onto the fibers relies on a sequence of preheating, sensitizing, activation and metallization, with important cleaning, rinsing, and drying stages also being included. The conditions used are detailed in Table 2. Table 2 summarizes the best working procedure used, which is based on improving different solutions used by some authors [18] -[22] .

The complete process of coating starts with the organic sizing and finish treatment of fibers in a muffle furnace for 10 min at 500˚C to eliminate the pyrolytic coatings around as―received fibers. After cleaning the fibers in an air furnace for 10 min at 500˚C, they were sensitized for 15 min under continuous stirring. Afterwards, fibers were filtered and cleaned with distilled water. In order to get catalytic surfaces, the fibers were activated under ultrasonic agitation. The heat cleaned fibers were first treated with glacial acetic acid to activate the surface, and then again activated using stannous chloride (12 g/l SnCl₂-2H₂O) and 40 ml/l of concentrated hydrochloric acid they were sensitized for different times under continuous stirring. Afterwards, fibers are filtered and cleaned with distilled water. In order to have catalytic surfaces, the sensitized fibers were exposed to an aqueous solution containing palladium chloride (0.2 g/l PdCl₂) and (2.5 m/l HCl) under ultrasonic agitation. This process, called activation, produces the formation of Pd sites on the fiber surface which allow the subsequent metallization with copper.

Metallization is produced by immersion of activated fibers into a solution containing CuSO₄-5H₂O as metal ion sources also held under agitation. Different metallization conditions have been tested, pH (12 & 13), time (2 min to 20 min) and temperature (40˚C & 50˚C), and continuous and crystalline coatings with homogeneous thickness have been obtained. The reactive volume used assures that the concentration of the diluted copper can

be considered constant during the deposition. The coatings obtained at different metallization temperature, time and pH values were studied by SEM and the thickness of the copper layer was determined in transversal cross section.

The effect of time of sensitization on the weight percent of copper deposited is shown in Figure 1.

Apparently, as the time for sensitization increases, the surface becomes more and more suitable to receive a coating. However, after about 15 minutes of sensitization, there appears to be no significant increase in the amount of copper deposited with further increase in sensitization time.

The effect of time of activation on the weight percentage of copper deposited is shown in Figure 2. Apparently, as the time for activation increases, the surface becomes more and more suitable to receive a coating. However, after about 15 minutes of activation, there appears to be no significant increase in the amount of copper deposited with further increase in activation time.

The effect of PdCl₂ concentration on the weight percent of copper deposited is shown in Figure 3. It can be seen that the changes in weight percent of copper deposited with a change in PdCl₂ concentration from 0.1 to 0.25 g/l of PdCl₂ is from 27 to 37.

Figure 4 shows that as the stirring time of basalt fibers in the electroless bath increases, the percent of copper deposited on the basalt fibers also increases up to a stirring time of about 30 min. The coating reaction starts as soon as the basalt short fibers activated by PdCl₂ are dispersed in the coating solution. Palladium metal at the surface of the basalt fiber acts as a catalyst. After the start of the coating reaction it proceeds autocatalytically. After putting the activated basalt fiber in the electroless bath, the bath should be stirred continuously to obtain a uniform coating of copper on the basalt fibers. The colour of the electroless solution changes progressively as

the reaction progress due to depletion of copper. The reaction is generally completed within about 30 min during which time about 35 to 36 wt% of copper is deposited on the basalt fiber due to depletion copper in the solution and the tendency for precipitation of free copper in the solution instead of a further build up of the coating.

Bath temperature must be kept below 50˚C because higher temperatures unstabilize the metallization solution. At this or higher temperatures dendritic growth of the coating is promoted. Using lower bath temperature uniform thickness can be achieved on the coating [23] .

To obtain any copper deposition, the pH of the metallization solution must be higher than 12. For pH 12 or higher the deposition rate increases as the pH increases, so shorter deposition times are needed to the same thickness. For pH 13, the minimum time needed to get a continuous copper coating is 2 min.

In the coating reaction, formaldehyde (HCHO) reduces CuSO₄, and copper deposited on the activated basalt fibers. It was found that after completion of the coating reaction, the pH of the solution had decreased to 10. After adding sodium hydroxide in an amount sufficient to maintain pH at 13, it was possible to coat copper on the basalt fiber by replenishing the solution with CuSO₄ and formaldehyde.

Figure 5 shows the surface characteristics of uncoated fibers (Figure 5(a)), of copper coated ones (Figure 5(b) and Figure 5(c)) under different metallization conditions, and Figure 5(d) shows the transversal section of copper coated fibres as observed with light microscopy. As it can be seen in the images, the thickness and morphology of the copper layers is highly dependent on the metallization conditions, mainly on sensitization time, activation time, metallization time, bath temperature and pH. Bath temperature must be kept below 50˚C because higher temperature unstabilizes the metallization solution. At this or higher temperatures dentritic growth of the coating (Figure 5(c)) is promoted. Using lower bath temperatures under the same pH conditions (pH 13) homogeneous thickness can be achieved on the coatings. To obtain any copper deposition, the pH of the metallization solution must be higher than 12. For pH 12 or higher the deposition rate increases as the pH increases so shorter deposition times are needed to get the same thickness. For pH 13, the minimum time needed to get continuous copper coating is 3 mins. After the first stage, the thin copper coating grows homogeneously in thickness with evidence of dentritic growth. The measured thickness of the copper coating for the conditions finally chosen (45˚C, pH 13 and 3 min)

The size of basalt fibers is very short (on the average from 0.5 to 1 mm), the key issue of development of the composite is the interaction between the matrix and the fibers. Because of this, the basalt fibers were coated with copper so that development emphasizes the interfacial aspects. In order to evaluate the coating morphology on the fibers, surface of the coated fibers were studied by SEM after the coating process. Evaluation of the effect of copper coating on the strength of the basalt fibers in as-received and after coating was carried out in different thickness of coating as shown in Figure 6. Tensile tests were performed using Single Fiber Test (SFT) method on universal testing machine with special grip (Figure 6(a)) according to ASTM D3379-75. Based on the introduced procedure of the test, a randomly selected basalt fiber was taken from the coated bundle. The gauge length was 25 mm and the test speed was 2 mm/min. The elementary fibers retrieved from the roving were stuck to paper windows (Figure 6(b)) and their diameter was measured on an optical microscope. Subsequently, the specimens were clamped to the testing machine, the paper window was cut and the fiber was torn (Figure 6(c)). Hundred tests were carried out for each experiment with an effective gauge length of 10 mm and a crosshead rate of 1.0 mm/min.

During the test, fibers were tested and the following values were measured: fiber diameter (d_f), fiber cross

section area (A_f), maximum force (F_max), extension at failure (Δ_max), specific elongation at failure (ɛ), tensile strength (σ_max) and Young modulus (E). The results can be seen in Table 3. The elongation and the Young modulus were calculated from the travel of the testing machine’s crosshead. The standard deviations also were calculated for each value.

Figure 7 shows the change of ultimate tensile strength of copper coated basalt fibers as a function of coating thickness. It has been observed from the graph that the tensile strength of copper coated basalt fiber decreases with increase in the coating thickness value. The tensile strength of copper coated basalt fibers has decreased from 2265 MPa in the coating thickness of 0.2 μm to 2152 MPa in the coating thickness of 0.3 μm. The main reason for this decrease in tensile strength of basalt fibers is due to the increment in cross-section of the coated fibers as a result of coating by copper. Copper coated tensile strength is much less than uncoated fibers. This conclusion could be driven from the curve, lowering the tensile strength as a result of increment of coating thickness although with different rates.

The morphology of original basalt fiber and coated basalt fiber was observed using a field emission scanning electron microscope (SEM) measurements were held with a JEOL JSM 6360―A model with a magnification capacity of 500×, 1000×, 2000× and accelerating voltage of 20 KV with working distance (WD) 10 mm and a spot size 58.Microscopes were equipped with analytical facilities (energy dispersive X-ray spectroscopy-EDS). The aim of the experimental plan is to find the important factors and combinations of Assistance of advanced measurement techniques has been taken to measure coating thickness of fiber at a micro-scale. The experimental results were confirmed by micro-structural studies using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of short basalt fiber obtained from the chemical test. Through the SEM images, the thickness of copper coating on the basalt fiber was measured. EDS scanning results shows the composition of material which was predominantly copper, from the large Si peaks; it also contains alumina and iron as shown in Figure 8(b). Figure 8(a) and Figure 8(b) show the EDS pattern of uncoated and copper coated basalt fiber respectively. The serious aggregations among the basalt fiber were observed due to the high chemical activity of copper atoms. The original basalt fiber exhibit a glossy surface. After sensitization and activation, the surface becomes dim. The EDS results confirmed the presence of Cu indicating a successful sensitization and activation process.