## 1 Introduction  
For the past decade, ceramic matrix composites (CMCs) modified with graphene or its derivatives have been attracting great attention as a novel class of materials for new energy storage devices and batteries [1; 2; 3], solid oxide fuel cells [4] and thermal barrier coatings [5; 6] with improved hardness, fracture toughness, bending strength and wear resistance [7]. Graphene additive limits grain growth in the ceramic matrix during sintering resulting in the composite reinforcement. Stabilized-zirconia-based composites are of particular interest since the introduction of graphene to the zirconia matrix results in mixed ionic-electronic conductivity [8; 9; 10]. The sintering step is a key factor to retain graphene additive in the zirconia matrix and to achieve the desired properties of CMCs. The fabrication of stabilized zirconia/graphene composites requires high-temperature sintering (1300-1600 \({}^{\circ}\)C), long exposure times and a vacuum or inert atmosphere [11; 12; 13]. To date, a number of zirconia/graphene composite sintering techniques have been proposed in the literature [7; 14]. They can be divided into two groups: equilibrium (convectional) sintering and non-equilibrium (field-assisted) sintering [15]. The first group of methods includes hot uniaxial pressing, hot isostatic pressing, pressureless sintering and some others [16; 17]. The sintering is realized through heat convection from the heating element to the sample, and it takes several hours to archive the full densification of a composite. The main advantages of the listed methods are (i) the possibility to reach a high degree of crystallinity; (ii) a wide range of starting oxide and non-oxide powder materials that can be sintered; and (iii) the dimensions and shape of the samples are variable and are limited only by furnace chamber size. However, the family of graphene materials is very sensitive to oxidation in the presence of residual oxygen up to \(10^{-3}\) atm at elevated temperatures (>300 \({}^{\circ}\)C) even when they are incorporated into a metal or ceramic matrix [10]. For that reason, sintering is usually conducted in a deep vacuum or in an argon atmosphere. However, successful sintering under these conditions requires a significant complication of the experimental setup (high-power vacuum pumps, oxygen control sensors, etc. ), which increases the cost and imposes restrictions on the number of simultaneously sintered samples. In [8; 9], fully yttria-stabilized zirconia (YSZ)/reduced graphene oxide (rGO) composites were sintered using both conventional sintering in air and sintering in a deep vacuum. It was shown that the rGO additive acts as a grain growth inhibitor, which is burned out completely during annealing in air. As a result, the refined microstructure of ceramics and coinciding temperature dependencies of grain and grain boundary conductivities in the whole temperature range were achieved. Vacuum annealing resulted in YSZ/rGO composites with high ionic conductivity and high crystallinity; however, the absence of electron conductivity was shown. Non-equilibrium sintering approaches including spark plasma sintering (SPS) are most commonly applied to obtain stabilized zirconia/graphene composites with desired mechanical and electrical properties [8; 10; 18]. This group of sintering techniques is based on the direct energy transfer from a source to the sample via an electric current or electromagnetic radiation. A high rate of temperature increase up to 1000 \({}^{\circ}\)C/min leads to the shortening of the sintering time to several minutes; this time, shortening limits the grain growth, allows sintering of the fully dense specimens and preserves graphene from oxidation. As it was shown in [8], SPS allows one to retain graphene in the zirconia matrix and leads to the mixed electronic-ionic conductivity of the composites.