Shaping of thermal protective properties of basalt fabric-based composites by direct surface modification using magnetron sputtering technique

3.1 Magnetron sputtering technique

Magnetron sputtering is a deposition technique that relies on a glow discharge under reduced pressure and between two mutually perpendicular fields: electric and magnetic [14,28]. In this process, the emitted electrons move along the spiral path in a direction perpendicular to the direction of both fields along the magnetic field force lines. The interplay between these two fields leads to electron confinement within an excitation zone. Consequently, there is a substantial surge in ionizing collisions within the gaseous atmosphere. This phenomenon significantly enhances plasma excitation and substantially increases plasma density within the volume adjacent to the magnetron. A strong electric field accelerates positively charged ions toward the cathode. The ions hitting the cathode surface cause its intensive sputtering. Magnetron sputtering can be carried out in an inert atmosphere, a result of which metallic coatings are produced. Magnetron sputtering can also be taken in an atmosphere that is a mixture of reactive gases, which makes coatings that are compounds (metals with carbon, oxygen, or nitrogen).

Basalt fabric was subjected to the deposition of coating process by the magnetron sputtering using Al, zirconium(iv) oxide, and titanium(iv) oxide. The magnetron sputtering coatings’ deposition was performed using an industrial vacuum unit URM 079 equipped with one magnetron plasma source. Detailed deposition parameters of coatings manufactured using the magnetron sputtering technique are given in Table 4.

3.2 SEM-EDS technique

SEM-EDS [29] was used to assess coating on basalt fabrics and determine their chemical composition. SEM-EDS is a powerful analytical technique commonly employed in materials science and various fields to examine the surface morphology of materials and identify the elements present in a sample. EDS is an integral part of SEM that provides information about the chemical composition of the material. When the focused electron beam of the SEM interacts with the sample, it causes the emission of characteristic X-rays from the elements present in the sample. This technique allows for detailed visualization of the coating thickness, texture, and any defects or irregularities. A JEOL JSM-6610 LV scanning electron microscope working in a low vacuum mode with an attached Oxford Instrument X-MAX 80 module was used.

3.3 Thermal conductivity

The thermal insulation properties of composites were assessed based on the thermal conductivity coefficient [30]. The thermal conductivity coefficient λ of homogenous materials expressed in W m⁻¹ K⁻¹ is given as follows:

where Q is the heat transmitted, A is the area, Δt is the temperature gradient, and h is the sample thickness.

Thermal conductivity is a material property that primarily determines the elementary functions of clothing, especially the user’s comfort. The thermal conductivity of materials can vary significantly depending on factors such as temperature, pressure, and composition. Alambeta (Sensora device) was used for testing basalt fabric-based composites [30]. A sample was placed between the upper plate with a temperature of 35°C (it should more or less correspond to the human skin temperature) and the perpendicular lower plate reaching the ambient temperature. Metal plates adhere to the tested sample with a pressure of approximately 200 Pa. The thermal conductivity coefficient (λ) is measured as the quantity of heat transported through the sample when the difference.

3.4 Resistance to contact heat

This testing aimed to assess the ability of basalt fabric and composites to resist contact heat. It is essential to evaluate their suitability for applications where protection against contact with hot surfaces is necessary. Two standards were taken into account. The primary standard for determining the contact heat resistance of the tested materials was ISO 12127-1:2016 [31]. Another standard was ISO 11612:2015 [32], which specified performance requirements for clothing to protect the user’s body from heat, flames, and other related hazards, excluding the hands. The test sample was placed on a calorimeter and then in contact with a heater heated to a temperature of 250°C. The threshold time t _t was measured from the moment the sample contacts the heater until the calorimeter temperature rises by 10°C. According to the standard [32], three efficiency levels (F1, F2, and F3) were considered:

• F1 when t _t falls within the range of [5.0, 10.0) s,

• F2 when t _t falls within the range of [10.0, 15.0) s,

• F3 for t _t greater than or equal to 15.0 s.

A longer threshold time indicates that the material can withstand contact with a hot surface for a more extended period before allowing significant heat transfer.

3.5 Resistance to radiant heat

This testing aimed to assess the ability of basalt fabric and composites to resist radiant heat. Radiant heat resistance is significant when evaluating materials intended for clothing that protect against heat and thermal radiation. Materials were tested to determine the radiant heat resistance based on standard [33]. A sample was placed on an appropriate holder and subjected to thermal radiation for a set time. An incident heat flux density of 20 kW/m² was taken into consideration. The time it took for the calorimeter temperature to rise by 24.0°C (± 0.2)°C was measured according to test method B. This temperature rise indicates that the user experienced second-degree burns. Four efficiency levels were considered based on estimated time RHTI₂₄ – the radiant heat transfer index [33]:

• C1 when RHTI₂₄ falls within the range of [7.0, 20.0) s,

• C2 when RHTI₂₄ falls within the range of [20.0, 50.0) s,

• C3 when RHTI₂₄ falls within the range of [50.0, 95.0) s,

• C4 for RHTI₂₄ greater than or equal to 95.0 s.

3.6 Similarity analysis of composite features

A geometric analysis of the similarity of composite features was performed to assess the impact of coating components content and coatings thickness on the thermal conductivity coefficient (λ), the resistance to contact heat (t _t), and the resistance to radiant heat (RHTI₂₄). Geometric similarity exists when two objects look exactly alike except for the fact that they are of different sizes. Similarity-based classifiers estimate the class label of a test sample based on the similarities between the test sample and a set of labeled training samples and between the pairwise of the training samples [34,35].

Let X be the sample space and G be the finite set of class labels. Let D: X × X → R be the similarity function. It is assumed that the pairwise similarities between n training samples are given as an n × n similarity matrix S whose (i, j)-entry is D(L _i , L _j ), where L _i ∈ X denotes the ith training sample and y _i ∈ G the corresponding ith class label (i = 1,2, …, n). Let the tangent distance-based classifier be the measure of similarity between pairwise of the training samples (variables). Tangent distance determines the relationship between the directions of vectors of variables L _i and L _j and is expressed as follows:

where r _i,j is the linear correlation coefficient between variables L _i and L _j , i, j = 1,2, …, n.

Variables whose vectors in the multidimensional feature space are parallel (r = 1) or antiparallel (r = −1) carry the same information, and their tangent distance equals 0. Variables whose vectors are orthogonal (perpendicular to each other; r = 0) carry entirely different information and have a tangent distance equal to infinity. For remaining cases, i.e., for variables whose value of the linear correlation coefficient is greater than −1 and less than 0 or greater than zero and less than 1, the tangent distance measure takes a deal from 0 to infinity. It means that the greater the distance between the variables, the less they carry common information.

4.1 One-layer coating composites

Preliminary studies of one-layer coating composite were conducted. Zirconia(iv) oxide and Al coatings of various thicknesses were deposited on the basalt fabric surface to assess the impact of coating thickness on resistance to contact and radiant heat of the composite. Two variants of the coating thickness were considered: 1.0 and 5.0 μm. The deposition parameters of the coatings manufactured using magnetron sputtering are given in Table 4. The measurement results of the resistance to contact heat for a temperature of 250°C and radiant heat for the one-layer coating composites are shown in Table 5 [20]. Additionally, results obtained for basalt fabric B are also juxtaposed for comparison.

The thermal conductivity coefficient of composites with one-layer coating increased compared to unmodified basalt fabric. The impact of the kind of deposited material (Al or ZrO₂) on the coefficient λ is not observed. The first efficiency level (F1) of protection against contact heat was achieved for the 5.0 μm thick ZrO₂ coating of basalt fabrics BZ2 for a contact temperature of 250°C. The first efficiency level (C1) of protection against radiant heat was achieved for samples B, BZ1, and BZ2. The second efficiency level (C2) of protection against radiant heat was achieved for both Al coating thickness of fabrics (BA1 and BA2). Even a thin coating of Al resulted in a significant improvement in the composite resistance to radiant heat. Basalt fabric-based composites with a two-layer coating (Al/ZrO₂) were manufactured to obtain materials protecting against contact and radiant heat.

4.2 Two-layer coating composites

Basalt fabric-based composites BAZ were manufactured. The magnetron sputtering parameters for two-layer coating (Al/ZrO₂) composites are given in Table 4. The coating thickness in a single layer not exceeding 2.0 μm was taken into account. SEM-EDS results for BAZ composite coating are shown in Table 6. The results of thermal properties measurements for two-layer coating composites are shown in Table 7.

The thermal conductivity decreased, meaning the two-layer composites are characterized by better thermal insulation than one-layer ones. It is a crucial fact that the inner layer was ZrO₂ – the material characterized by low thermal conductivity compared to Al. The outer Al layer increases the thickness of the coating. The first efficiency level (F1) of protection against contact heat was achieved for BAZ1 and BAZ4 composites. The first efficiency level (C1) of protection against radiant heat was achieved for BAZ1, and the second (C2) was reached for composites BAZ2, BAZ3, and BAZ4. It means that the protective properties of the composites against radiant heat were visibly improved compared to one-layer coating composites. BAZ composites showed slightly better protection against contact heat.

The similarity analysis (I) was carried out for variables: L ₁ – the weight percentage of zirconium dioxide (denoted as ZrO₂ in Table 8), L ₂ – the weight percentage of aluminum (denoted as Al in Table 8), L ₃ – the total weight percentage of aluminum and zirconium dioxide (denoted as Al/ZrO₂ in Table 8), L ₄ – the thermal conductivity coefficient λ, L ₅ – the threshold time t _t, and L ₆ – the radiant heat transfer index RHTI₂₄. Results of the similarity analysis (I) of the composite features ordered from the smallest tangent distance (equation (1)) to the largest one between variables L _i and L _j are shown in Table 8. A positive “+” and negative “−” correlation for the chosen pairs of variables is also given.

The shortest tangent distance was noticed between the weight percentage of ZrO₂ observed on the coating surface and resistance to radiant heat RHTI₂₄. A strong relationship was confirmed (r _1,6 = −0.994), which indicates that the higher the weight percentage of ZrO₂ detected on the surface, the lower the composite resistance to radiant heat. At the same time, the analysis results indicate that the resistance to radiant heat will increase with the increase in Al (r _2,6 = 0.833). The comparable tangent distance between Al and resistance to contact heat t _t was also noticed. Thus, the increase in the weight percentage of Al being heat-dissipating but also thermal conductivity material causes the parameter t _t improvement (r _2,5 = 0.817). It means that the inner ZrO₂ layer also has a significant impact on the protective properties of the composite against contact heat. Both Al and ZrO₂ have an impact on the thermal conductivity coefficient λ. However, a moderate positive correlation for Al and a negative one for ZrO₂ was found.

4.3 Two-layer coating composites enriched with ZrO₂

The similarity analysis (II) was carried out in order to select coating thickness that will improve the protective properties of composites. The following variables were chosen: L ₁ – the ZrO₂ coating thickness (denoted as ZrO₂ in Table 9), L ₂ – the Al coating thickness (denoted as Al in Table 9), L ₃ – the thermal conductivity coefficient λ, L ₄ – the threshold time t _t, and L ₅ – the radiant heat transfer index RHTI₂₄. The results of the analysis are given in Table 9.

The shortest tangent distance means the best similarity and strong linear relationship (r _2,5 = 0.984) was observed between Al coating thickness and the radiant heat transfer index RHTI₂₄. The thickness of ZrO₂ and Al coatings are also important from the point of view of improving t _t. The thicker the zirconium(iv) oxide and/or Al coating, the higher the threshold time t _t value, i.e., the effectiveness of composite protection against contact heat. However, the linear correlation is much stronger for ZrO₂ than for Al (r _1,4 = 0.832 and r _2,4 = 0.588, respectively). The moderate impact of the ZrO₂ and Al coatings thickness on the thermal conductivity was found for BAZ composites, with a negative correlation between variables λ and ZrO₂ and a positive correlation between λ and Al. Compatibility between thermal conductivity and reliability in strength and heat resistance have to be ensured when using Al. It was decided that not exceeding 1.0 μm thick Al outer coating of the new composites will be considered. Therefore, three Al coating thicknesses were taken into account 0.3, 0.5, and 1.0 μm (Table 4).

It was observed that zirconium(iv) oxide coating cracks as a result of aging in ambient conditions. Thus, it was decided to enrich the inner layer of the composites. Titanium(iv) oxide inhibits the coating aging process and is characterized by a low thermal conductivity coefficient. Titanium(iv) oxide was considered in combination with zirconium(iv) oxide. It was expected that an enrichment of the composites inner layer with TiO₂ would improve the resistance to contact heat. Two coating thicknesses of ZrO₂ and TiO₂ mixture (50/50) were taken into account: 1.0 and 2.0 μm (Table 4).

New basalt fabric-based composites, BAZT, were manufactured with the use of magnetron sputtering in accordance with the adopted assumptions regarding the coating thicknesses given in Table 4. SEM-EDS results for BAZT composites coating are shown in Table 10. The results of the thermal parameters of the composites are shown in Table 11.

Based on values of the thermal conductivity coefficients, it was stated that the thermal insulation properties of BAZT composites remained generally unchanged. It was found that the composites still had the first efficiency level (F1) of protection against contact heat. Nevertheless, the threshold time has increased, especially for BAZT2 and BAZT3. The second efficiency level (C2) of protection against radiant heat was achieved for all BAZT composites.