Effect of Heat Treatment on Electrical Insulation of Strain Sensors for Aluminum Cast Parts ^†

Abstract

This work presents the effect of thermal treatment on the electrical insulation of strain sensors on aluminum substrates. The sensors are meant to be embedded into cast aluminum parts, which are heat-treated for strengthening via precipitation hardening. For sensor manufacturing, thick film materials are used for the electrical insulation and its connection tracks, whereas sensing platinum structures are produced by sputtering. The effectiveness of different insulation thicknesses was tested for a treatment regime of 7 h at 535 °C, which matches solution heat treatment conditions as the most demanding part of the precipitation hardening process. The results showed that insulation is partially lost after treatment, and six consecutive insulating layers are required to produce an insulation capable of withstanding an extended heat treatment.

1. Introduction

Recent research towards the integration of sensing elements into metal parts [1,2] opens the possibilities of smart sensing in structural elements that could be produced on a high scale, for example by metal casting [3,4]. In this scenario, the sensor materials should be robust enough to withstand the integration process [4,5] but also the heat treatments, which are usually performed to improve the mechanical properties of metal parts. For example, a typical T6 treatment for aluminum alloy [6] includes a solution heat treatment step at 535 °C for several hours, which can promote the diffusion of material within and among sensor and insulation material layers, causing degradation of sensor materials or fracture caused by grain growth [7]. Therefore a proper design of the electrical insulation of the sensors is required.

2. Materials and Methods

An aluminum alloy AlMg3 plate with a thickness of 2 mm was used as substrate for the sensors. The aluminum plate was coated by manual screen printing and sintering for 15 min at 570 °C of the paste IP6080A (Heraeus Deutschland GmbH & Co. KG, Hanau, Germany). The samples were produced with 2, 3, and 6 consecutive bottom layers; the thickness of a single coating layer after sintering was ~20 µm. Platinum meander structures (350 nm thick) were produced on top of the insulation layers by DC sputtering using a custom silicon shadow mask. A Titanium layer (40 nm thick) was used as an adhesion promoter between the platinum and the bottom insulation. Connecting tracks between platinum structures and contact pads were fabricated using the silver paste C8829D (Heraeus Deutschland GmbH & Co. KG, Hanau, Germany) by manual screen printing and sintering (570 °C) of a single layer. The sensors were coated with two top-layers of the same IP6080A paste, each layer was sintered at 570 °C. The samples were then thermally treated in a batch furnace (type N250/85HA, Nabertherm GmbH, Lilienthal, Germany) for 7 h at 535 °C. The sensor pads were considered electrically insulated from the substrate if the electrical resistance was above the range of the digital multimeter Keithley 6500.

3. Discussion

A total of 78 sensors were produced on an aluminum plate per each bottom insulation condition, i.e., 2, 3, and 6 layers. The fabrication schema and the example of an actual sensor (without top isolation) are shown in Figure 1a, whereas the electrical insulation rate is plotted in Figure 1b.

Measurements showed that the samples with only two bottom insulation layers were not properly insulated, as only 27 % of the sensors showed sufficient resistance levels between conductive paths and substrate prior to the heat treatment, and only 16% retained this resistance after thermal processing. The samples with three bottom insulation layers (manufacturer’s recommendation) achieved 98% insulation before heating, and about 92% of the insulation layers withstood the heat treatment in this case. Of the samples with 6 bottom insulation layers, 100% survived both the sintering process and the heat treatment. This indicates that a temperature-resistant insulation can be achieved by increasing the number of insulation layers. This result may be explained by local defects occurring in the individual insulation layers during thermal processing when assuming that the likelihood of a direct superposition of such defects decreases with increasing number of layers. These results will be considered in future fabrication of sensors for embedding into aluminum casting parts, and their subsequent heat treatment.