Does A Thicker Coating Mean Better Insulation? A Selecting Guide

Many users may have the following questions when selecting an insulated bearing:

"Does a thicker coating mean better insulation?"
"Is a thinner coating not safe enough?"

The answer is: not necessarily.

In theory, a thicker coating can offer higher insulation resistance and breakdown voltage, providing better protection against high voltages. However, in practical applications, coating thickness should never be a one-size-fits-all decision. Excessive thickness may cause dimensional deviations and negatively impact heat dissipation, while overly thin coatings can compromise insulation performance.

This article will provide a detailed guide on how to select the optimal coating thickness based on your specific application needs.

Insulating Coating Working Principle

The insulation performance of electrically insulated bearings largely depends on the ceramic insulating coating applied to their surfaces. This coating is typically made of aluminum oxide (Al₂O₃) ceramic material and is firmly bonded to the outer or inner ring of the bearing using plasma spraying or similar processes. It forms a high-hardness, high-resistivity insulation barrier.

In addition to its insulating function, the ceramic coating also provides a certain level of corrosion resistance and wear resistance, helping extend the service life of the bearing in harsh environments such as high humidity, salt spray, and dusty conditions.

Ceramic materials possess excellent dielectric strength. Within a reasonable thickness range, the insulation coating can typically withstand up to 1000V or even higher of industrial frequency or pulse voltage without dielectric breakdown. By adjusting the coating thickness and spray parameters, the breakdown voltage level can be further improved to meet the requirements of various voltage applications.

Although the ceramic coating is highly rigid, its thermal expansion coefficient differs from that of bearing steel. In TFL’s coating design, substrate pretreatment, spray process optimization, and post-treatment techniques are carefully engineered to ensure strong adhesion and mechanical compatibility between the coating and the metal substrate, enabling long-term operation under vibration, thermal expansion, and alternating temperature conditions.

Is A Thicker Insulating Coating Always Better?

The answer is no. A thicker insulation coating is not always better. Bearing designers and manufacturers typically control the coating thickness to achieve a balance between insulating performance and mechanical properties. In practical applications, excessive thickness can lead to counterproductive effects:

1. Diminishing Returns (Threshold Effect): Intuitively, one might think a thicker coating means better insulation and stronger mechanical properties (like impact resistance). This is true to a certain extent, as a thicker dielectric layer can typically withstand higher voltages and has a longer creepage distance. However, the insulating strength gained from the coating is limited. Once the coating thickness has reached the necessary breakdown voltage, the increase in insulation effectiveness from adding more thickness is marginal. The thicker the coating, the more difficult and costly the spraying process becomes. At the same time, a thick coating can be more prone to defects like cracks and pores during manufacturing, which compromises its reliability.
2. Impact on Dimensional Accuracy and Installation: As we know, a bearing’s fit tolerance and surface roughness are critical parameters for successful installation. When the coating is too thick, the bearing’s dimensions may exceed its tolerance range, affecting its mating fit with the shaft or housing. This can negatively impact the bearing’s operational stability or even lead to installation failure.
3. Reduced Heat Transfer: Ceramic coatings inherently have a low thermal conductivity. An excessively thick coating will impede the bearing’s ability to transfer heat from the inside out. This can cause the bearing to heat up faster during operation, potentially affecting the performance of the lubricant and the overall bearing life.
4. Impact Resistance: The bond strength between the coating and the steel substrate is crucial in the spraying process. If the coating is too thick, the internal stress can increase. A rigid coating that is too thick may actually be more susceptible to brittle fracture or spalling under severe impact because it lacks sufficient deformation capability.

Is A Thinner Insulating Coating Always Better?

Since a thicker coating isn’t always better, should one aim for the thinnest possible coating? The answer is still no.

As previously mentioned, the insulating performance of an electrically insulated bearing largely depends on its ceramic surface coating. The coating should not be made as thin as possible; it must meet a fundamental safety threshold.

1. Risk of Dielectric Breakdown: The most direct consequence of a coating being too thin is its failure to provide sufficient dielectric strength to effectively block the shaft current. At higher voltages, an overly thin coating is susceptible to electrical breakdown. This causes the bearing to lose its insulating function and may lead to permanent damage to the coating itself.
2. Manufacturing and Uniformity Challenges: It is difficult to ensure uniformity and porosity-free coverage when manufacturing extremely thin coatings. Ceramic coatings are applied via high-temperature spraying. A very thin layer is more likely to develop microscopic pores, pinholes, or micro-cracks. These defects act as potential pathways for electrical current, significantly reducing the overall insulating reliability. Furthermore, thinner coatings demand higher precision in the spraying process, temperature control, and material purity, making it more challenging to guarantee high-quality insulation performance across every batch.
3. Susceptibility to Mechanical Damage: The primary weakness of the ceramic coating is its brittleness. A thinner coating is more easily damaged by minor impacts, scratches, or improper handling during bearing installation, removal, or routine maintenance. More importantly, an overly thin coating possesses insufficient mechanical properties. It may not be able to withstand the impulses or thermal shocks encountered in the bearing’s operating environment, leading to flaking or cracking. In conditions involving salt spray, high humidity, or high-frequency vibration, a thin coating is more prone to degradation. Once the surface coating is compromised, the insulating function fails immediately.

How to Choose the Best Coating Thickness

The optimal thickness is the best thickness. In practice, there is no one-size-fits-all standard answer for the coating thickness of an electrically insulated bearing, and you should not blindly pursue either the thickest or the thinnest option. The ideal thickness is a balance among insulation performance, dimensional accuracy, thermal conductivity, service life, and cost.

Before determining the thickness, it’s best to consider the following factors:

Insulation Strength (Dielectric Withstand Voltage)

Insulation performance is the number one priority. You need to understand your equipment’s maximum common-mode voltage or peak shaft voltage. Select a bearing with a dielectric rating that is higher than the peak shaft voltage, allowing for an adequate safety margin.

• Low Voltage (e.g., tens of volts): A relatively thin coating (e.g., 50–100 µm) is often sufficient.
• High Voltage (e.g., hundreds of volts or higher) / High Current: A thicker coating is necessary to provide higher dielectric strength and a longer insulation life for long-term reliability. A thickness of 200 µm, 300 µm, or even more may be required.

Operating Temperature

Ceramic coatings offer good high-temperature resistance. A slightly thicker coating can provide better thermal stability and more durable insulation at high temperatures. However, as previously mentioned, an overly thick coating hampers heat dissipation. Therefore, in high-temperature environments, the thickness must be considered alongside heat management needs. Bearings in low-temperature environments generally have less stringent thickness requirements.

Impact and Load

If your bearing operates smoothly or under light load, the thickness requirement is relatively low. However, if the working conditions involve heavy loads, vibration, or high impacts, the coating must possess good mechanical strength and resistance to spalling. In such cases, a greater thickness is necessary. Note that an excessively thick, rigid coating may still pose a risk of brittle fracture under extreme impact, requiring a balanced consideration.

Operating Environment

The ceramic coating itself offers some chemical corrosion resistance.

• If specific corrosive media are present, the coating thickness may need to be increased to provide longer protection.
• In environments with high humidity, dust, or conductive contaminants, a thinner coating is more easily penetrated by contaminants. A thicker coating provides a longer creepage distance, enhancing surface insulation performance and resistance to contaminants.

Long-Term Reliability

If you require the bearing to maintain excellent insulation performance over a long period, choosing a coating that is slightly thicker than the minimum requirement is a wise choice.

Recommended Coating Thickness Ranges (Reference Table)

TFL’s Coating Solutions

The ideal coating thickness must be determined based on the bearing’s actual operating conditions. Whether the thickness is too great or too thin, it can introduce unnecessary risks or costs.

When selecting a bearing, it is essential to thoroughly review all parameters of your application, including:

• Maximum Voltage
• Expected Current
• Operating Temperature Range
• Load Type
• Environmental Conditions
• Expected Service Life

TFL offers a wide range of coating thicknesses, from 50 to 500 µm.

If you have any questions about selecting the specific thickness for your application, please don’t hesitate to contact our technical experts. TFL has extensive application experience and specialized knowledge to recommend the most economical, reliable, and suitable coating thickness solution based on your exact needs. We consider not only the insulation performance but also the bearing’s overall operating efficiency and long-term stability.

TFL welcomes your inquiries at any time! You can start an online chat using the CHATTING BUTTON on the right, send an email to info@insulated-bearings.com, or click on “Contact Us” to get in touch. We will respond to all inquiries within 48 hours.