Aluminium Oxide Coatings for Electrical Insulation of Hard Carbon Thin Film Sensors
Kolehmainen, Jukka (2023)
Kolehmainen, Jukka
Tampere University
2023
Teknisten tieteiden tohtoriohjelma - Doctoral Programme in Engineering Sciences
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
Väitöspäivä
2023-10-06
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-03-3040-8
https://urn.fi/URN:ISBN:978-952-03-3040-8
Tiivistelmä
Thin film sensors embedded into the component level of machines are future solutions to bring functionality and control to automotive and industrial applications, such as autonomous control functions and robotics. Piezoresistive hard carbon thin film sensors represent this kind of approach to the function of sensing temperature and strain conditions on component level, as they possess a high sensitivity for measurement of physical actions with components of small dimensions and tight tolerances.
Piezoresistive hard carbon sensors, when applied on steel substrates, have to be isolated from the electrically conductive substrate. Two basic requirements were set for the necessary electrical insulation. The first requirement was a minimum resistivity of 108 Ωm over the temperature range between -10 °C and +100 °C under ambient air conditions of 20% RH, and the second one was the breakdown voltage of the insulating layer up to 200 VDC. However, there were several open questions in the leakage current measurements, related to the stability of the aluminium oxide layers when determining the resistivity of the coatings when they were exposed to changing ambient conditions. Much of the work in the study consisted of leakage current measurements performed as step response type measurements of 30 min transient periods in consecutive steps.
Atmospheric plasma spray (APS), high velocity oxygen fuel (HVOF) and physical vapour deposition (PVD) aluminium oxide coatings were investigated as-deposited, without additional post-treatment. All the APS and HVOF coatings fulfilled the set requirements at coating thicknesses over 40 µm. The practical range for thermal spray coatings was between 40–100 µm with resistivity between 1010 and 1011 Ωm. A slightly higher resistivity of between 5x1010–1012 Ωm was measured from suspension high velocity oxygen fuel coatings (S-HVOF). Thicker APS coatings demonstrated a much longer humidity-related current drift time in leakage current to decrease to a constant level.
Resistivity in the PVD aluminium oxide coatings varied between 4x109–1012 Ωm, which was influenced by the high defect density observed in the resistance screening of the samples. Practical minimum thickness for PVD aluminium oxide coatings was estimated to be between 2 and 3 µm to fulfil the requirement of a voltage duration of 200 V. Drift caused by humidity was also observed with the PVD aluminium oxide coatings but the stability in the leakage current was reached much faster than with the thermal spray coatings.
The thin PVD films made of Ti6Al4V as counter electrodes in the determination of leakage current, and as the contact pads of the piezoresistive hard carbon thin films highlighted the influence of surface roughness and porosity of the insulating layers under variable ambient conditions, especially with thermal spray insulating coatings. The Ti6Al4V films deposited were conformal and conductive enough for measurements but not pinhole-free, which allowed the permeation of moisture. The advantage of this was the possibility to study the ionic space charge polarization of water in the aluminium oxide layers with leakage current measurements under different temperature and humidity conditions. The polarization phenomena of aluminium oxide coatings were complex, starting from adsorbed water in chemisorbed state, changing to physiosorbed and finally to ionic movement in the electric field and anodic corrosion on the mild steel substrates. Direct current measurement cannot distinguish between surface and bulk conductivity but the rapid changes in leakage current seen close to the dew point temperatures implied that the huge changes measured in resistivity were related more to changes in surface resistivity caused by the condensed water on the hydrophilic γ-aluminium oxide surfaces of the thermal spray aluminium oxide coatings. This is also supported by the fact that the APS coatings without the use of a sealant showed a similar change in resistance close to dew point conditions at 80% RH. The adsorbed water in the aluminium oxide coatings caused more long-term drift influenced by the effect of ionic movement in the space charge polarization.
Measurements of the thermistor beta value and gauge factor of the hard carbon thin film sensor elements on thermal spray coatings as-deposited were new and the results were encouraging for future work with embedded hard carbon sensors. The conformity of the Ti6Al4V contact electrode and hard carbon layers was good over rough aluminium oxide coatings. The conductivity and functionality of the sensor layers were achieved by compensating for the thickness of the thin film layers. The good adhesion of the hard carbon layer on the aluminium oxide dielectric coatings was also a positive result seen in the study of mechanical fatigue resistance. It was observed that the roughness of the thermal spray aluminium oxide coatings brought additional improvement by means of mechanical adhesion to the PVD sensor thin film layers.
In this study it has been demonstrated successfully that it is possible to deposit functional piezoresistive hard carbon thin film sensors on steel substrates by using as-deposited APS and HVOF thermal spray aluminium oxide coatings for electrical insulation between the sensing element and the substrate. The beta value 1070 ± 15 and gauge factor 5.83 ± 0.46 of such sensors are comparable to those earlier measured on a polyimide substrate.
The scientific novelty of the thesis is also the extensive study of moisture-induced polarization effects in porous aluminium oxide structures performed with the direct current measurement concept developed for the measurement of leakage current in electrical insulating coatings.
Piezoresistive hard carbon sensors, when applied on steel substrates, have to be isolated from the electrically conductive substrate. Two basic requirements were set for the necessary electrical insulation. The first requirement was a minimum resistivity of 108 Ωm over the temperature range between -10 °C and +100 °C under ambient air conditions of 20% RH, and the second one was the breakdown voltage of the insulating layer up to 200 VDC. However, there were several open questions in the leakage current measurements, related to the stability of the aluminium oxide layers when determining the resistivity of the coatings when they were exposed to changing ambient conditions. Much of the work in the study consisted of leakage current measurements performed as step response type measurements of 30 min transient periods in consecutive steps.
Atmospheric plasma spray (APS), high velocity oxygen fuel (HVOF) and physical vapour deposition (PVD) aluminium oxide coatings were investigated as-deposited, without additional post-treatment. All the APS and HVOF coatings fulfilled the set requirements at coating thicknesses over 40 µm. The practical range for thermal spray coatings was between 40–100 µm with resistivity between 1010 and 1011 Ωm. A slightly higher resistivity of between 5x1010–1012 Ωm was measured from suspension high velocity oxygen fuel coatings (S-HVOF). Thicker APS coatings demonstrated a much longer humidity-related current drift time in leakage current to decrease to a constant level.
Resistivity in the PVD aluminium oxide coatings varied between 4x109–1012 Ωm, which was influenced by the high defect density observed in the resistance screening of the samples. Practical minimum thickness for PVD aluminium oxide coatings was estimated to be between 2 and 3 µm to fulfil the requirement of a voltage duration of 200 V. Drift caused by humidity was also observed with the PVD aluminium oxide coatings but the stability in the leakage current was reached much faster than with the thermal spray coatings.
The thin PVD films made of Ti6Al4V as counter electrodes in the determination of leakage current, and as the contact pads of the piezoresistive hard carbon thin films highlighted the influence of surface roughness and porosity of the insulating layers under variable ambient conditions, especially with thermal spray insulating coatings. The Ti6Al4V films deposited were conformal and conductive enough for measurements but not pinhole-free, which allowed the permeation of moisture. The advantage of this was the possibility to study the ionic space charge polarization of water in the aluminium oxide layers with leakage current measurements under different temperature and humidity conditions. The polarization phenomena of aluminium oxide coatings were complex, starting from adsorbed water in chemisorbed state, changing to physiosorbed and finally to ionic movement in the electric field and anodic corrosion on the mild steel substrates. Direct current measurement cannot distinguish between surface and bulk conductivity but the rapid changes in leakage current seen close to the dew point temperatures implied that the huge changes measured in resistivity were related more to changes in surface resistivity caused by the condensed water on the hydrophilic γ-aluminium oxide surfaces of the thermal spray aluminium oxide coatings. This is also supported by the fact that the APS coatings without the use of a sealant showed a similar change in resistance close to dew point conditions at 80% RH. The adsorbed water in the aluminium oxide coatings caused more long-term drift influenced by the effect of ionic movement in the space charge polarization.
Measurements of the thermistor beta value and gauge factor of the hard carbon thin film sensor elements on thermal spray coatings as-deposited were new and the results were encouraging for future work with embedded hard carbon sensors. The conformity of the Ti6Al4V contact electrode and hard carbon layers was good over rough aluminium oxide coatings. The conductivity and functionality of the sensor layers were achieved by compensating for the thickness of the thin film layers. The good adhesion of the hard carbon layer on the aluminium oxide dielectric coatings was also a positive result seen in the study of mechanical fatigue resistance. It was observed that the roughness of the thermal spray aluminium oxide coatings brought additional improvement by means of mechanical adhesion to the PVD sensor thin film layers.
In this study it has been demonstrated successfully that it is possible to deposit functional piezoresistive hard carbon thin film sensors on steel substrates by using as-deposited APS and HVOF thermal spray aluminium oxide coatings for electrical insulation between the sensing element and the substrate. The beta value 1070 ± 15 and gauge factor 5.83 ± 0.46 of such sensors are comparable to those earlier measured on a polyimide substrate.
The scientific novelty of the thesis is also the extensive study of moisture-induced polarization effects in porous aluminium oxide structures performed with the direct current measurement concept developed for the measurement of leakage current in electrical insulating coatings.
Kokoelmat
- Väitöskirjat [4946]