The thermoelectric effect, also called Seebeck effect, describes the generation of a voltage due to a temperature difference between the electrical junctions of two different conductors. The voltage is dependent on the difference in the material’s Seebeck coefficients and is approximately proportional to the temperature difference. Sensors based on this effect are called thermocouples. Weinert et al. [
14] used micro-milled masks and an arc PVD process for coating the rake face of ceramic silicon nitride turning inserts with three conductive pairs of Ni and NiCr paths, and they were tested during the dry turning of cast alloy GG25. However, the application of thin metallic PVD layers on silicon nitrides features an insufficient adhesion due to the lack of diffusive bonding during coating and the different thermal expansion coefficients of materials during cooling after the coating process. The sensors were not protected against chips or flowing material, so the cutting depth was selected according to that, measuring temperatures up to
\(150\,^{\circ }\hbox {C}\). Biermann et al. [
15] continued working with the same sensor design, including a wear-protection layer of
\(\hbox {Al}_{2}\hbox {O}_{3}\) above the sensor paths. The authors investigated the effect of the mask thickness, the path width, and the path geometry, proving that they directly affect the sensor performance. The sensors were tested during the turning process of gray cast iron material GJL-250 and GJL-600, measuring temperatures up to
\(200\,^{\circ }\hbox {C}\). The sensors were calibrated using two methods: By heating the workpiece during the turning process with a torch and measuring the temperature in the tool with thermocouples, as well as by submerging the tool tip in an oil bath with controlled temperature. Tillmann et al. [
16,
29] using the same manufacturing strategy, studied the effect of the monolayer thickness and coatings composition on the wear properties of the PVD layer. It was concluded that the Cr-CrN material combination presents higher wear resistance and micro hardness. The main problems of the thin-film thermocouples were also investigated: Delamination, flake, and geometrical inaccuracies as the effect of the ground material surface roughness on the adhesion of the coating. Werschmoeller et al. [
17] investigated the capability of high-temperature materials such as tungsten-5% rhenium and tungsten-26% rhenium for the manufacturing of thermocouples and tested them in orthogonal cutting experiments of
\(\hbox {AISI-O}_{2}\). The sensors were integrated into the cutting tool by diffusion bound and placed on the flank face, parallel to the cutting process plane. The sensors measured temperatures up to
\(900\,^{\circ }\hbox {C}\) and showed a response time of fewer than 150 ns. Kesriklioglu et al. [
18] developed a thin-film sensor composed of a thin layer of chromium sputtered on the rake face of a WC-Co insert as an adhesion promoter to form a compatible interface between the ground material and one
\(\hbox {Al}_{2}\hbox {O}_{3}\) dielectric layer. K-Type thin-film thermocouple conductive paths were located
\(30\,\upmu \hbox {m}\) to the cutting edge. Another coating followed the thermocouple fabrication process with
\(\hbox {Al}_{2}\hbox {O}_{3}\) to prevent a shortcut between the measurement junction and the AlTiN protective layer. The sensors were tested during the oblique interrupted cutting test with 12L14 steel. Basti et al. [
11] also manufactured a thin-film sensor system made of Ni-(Ni-Cr) thermocouples protected by an
\(\hbox {Al}_{2}\hbox {O}_{3}\)+AlN or TiN layer and insulated with a layer of
\(\hbox {HfO}_{2}\) using PVD and photolithography on the rake face of alumina tools, under the chip-tool contact surface at 0.3 mm distance from the cutting edge. The sensors were tested during orthogonal cutting of A6061-T6 aluminum alloy. The response time was 300 ms and measured maximum temperatures of
\(620\,^{\circ }\hbox {C}\). Shinozuka et al. [
19,
20] also deposited thin-film sensors made of seven pairs of built-in micro Cu-CuNi thermocouples on the rake face of carbide tools
\(600\,\upmu \hbox {m}\) away from the cutting edge. Still, they made micro-grooves on the rake face of alumina tools with ultrasonic machining to embed the conducting paths through electroless plating. The Cu-Ni thermocouples were then protected against wear with a
\(\hbox {Si}_{3}\hbox {N}_{4}\) coating. The sensors were tested during the longitudinal turning of A5056 aluminum alloy. Since the micro thermocouples were set in the grooves, the sensors’ endurance was higher than those placed on a plane and polished rake face. Sugita et al. [
21] continued with the idea of an array of micro thermocouples integrated on the tool’s rake face and placed into grooves. The groves were manufactured utilizing laser milling and coated with an insulating film of
\(\hbox {Al}_{2}\hbox {O}_{3}\) and with conductive paths of Cr. On the measuring point, there is no insulating coating. Thus the Cr comes into contact with the cutting tool ground material, made of cemented carbide, leading to a (WC-Co)-chromium (Cr) thermocouple. The WC-Co itself forms the cutting tool, and the sensor has a characteristic feature that it can be miniaturized. WC-Co is one of the main components of a cutting tool, and it produces a high negative thermoelectric power. The sensors were tested during the turning of MC Nylon. Cui et al. [
22] placed
\(\hbox {SiO}_{2}\) insulating film, NiCr/NiSi thermocouple film and
\(\hbox {SiO}_{2}\) protecting film on the surface of HSS substrates. Bobzin et al. [
23] developed a thin-film sensor system composed of a TiAlN+
\(\hbox {Al}_{2}\hbox {O}_{3}\) insulation layer deposited on the cutting tool made of 1.2343 hot work tool steel, a Ni-NiCr thermocouple and an
\(\hbox {Al}_{2}\hbox {O}_{3}\) wear protective and insulating layer. A novel characterization method was presented using a heat plate and measuring the temperature with an external thermocouple and an infrared camera. Li et al. [
24] manufactured six pairs of Ni-Cr/Ni-Al conductive paths on grooves on the rake face of tungsten carbide-cobalt cutting inserts in the chip-tool contact area and protected them with a film of
\(\hbox {Si}_{3}\hbox {N}_{4}\). In following investigations of the same working group C-type sensors consisting of tungsten-26%rehenium (W/Re26) and tungsten-5%rehenium (W/Re5) were tested on PCBN tools for machining of
\(\hbox {Ti}_{6}\hbox {Al}_{4}\hbox {V}\) and their durability was investigated, resulting in an increase in tool life compared to thin film sensors placed on a flat surface without grooves [
25].