Epoxy resin coating
In the field of industrial protection, epoxy resin coatings are often used to protect metal components. When the thickness of the epoxy resin coating is relatively thin, such as controlled at around 50-100 microns, due to the relatively limited cross-linking structure formed after curing, local deformation is prone to occur when subjected to external forces, and the overall hardness is relatively low. Using a hardness tester to test the Rockwell hardness (HR) may only be around 60-70.
As the coating thickness gradually increases to 200-300 microns, the resin inside the coating has more space to further improve the cross-linking network. The molecular chains support each other and entangle more tightly, which can better disperse external forces and resist deformation. At this time, the hardness will increase, and the Rockwell hardness (HR) can reach around 80-90.
However, if the coating thickness continues to be blindly increased beyond 500 microns, due to the shrinkage stress generated by the epoxy resin coating during the curing process, excessive stress accumulation inside the thick coating can easily lead to defects such as cracking and hollowing, which can actually damage the integrity of the coating, causing fluctuations or even a decrease in hardness measurement values, and greatly reducing the actual performance of the coating.
Ceramic coating (taking alumina ceramic coating as an example)
Aluminum oxide ceramic coatings are widely used for the protection of high-temperature components such as aircraft engine blades. When thin alumina ceramic coatings are prepared through processes such as thermal spraying, with a thickness of approximately 100-150 microns, the bonding between coating particles is relatively insufficient, and the overall coating is thin, with limited ability to resist external forces such as friction and compression. Its Vickers hardness (HV) may be around 1000-1200.
When the coating thickness is increased to 300-400 microns, more ceramic particles pile up and sinter together, forming a denser structure. The internal grain boundaries and other structures of the coating are more complete, enhancing its resistance to external forces. The Vickers hardness (HV) can be increased to around 1500-1800.
But when the coating thickness further increases to over 800 microns, on the one hand, the ceramic particles sprayed on during the thermal spraying process are difficult to fully integrate into the existing coating structure, which can easily lead to problems such as pores and delamination; On the other hand, the differences in thermal and mechanical stresses between the coating and the substrate due to excessive thickness can also increase, leading to a decrease in coating adhesion, peeling, and other conditions, which in turn affect hardness. The Vickers hardness (HV) value may become unstable or even decrease.
Hot dip galvanized coating
Hot dip galvanized coating is commonly used for anti-corrosion of steel used in construction. When the hot-dip galvanized layer is first formed, its thickness is relatively thin, possibly between a few micrometers and tens of micrometers. For example, when it is 10-20 micrometers, the zinc layer is softer and has a lower hardness, with a Brinell hardness (HB) of about 30-40. It mainly serves to isolate the steel from the external environment and provide preliminary corrosion protection.
As the hot-dip galvanizing time prolongs and the thickness of the galvanized layer increases to 50-100 microns, the crystallization of the zinc layer becomes more complete, the structure becomes relatively dense, and the hardness also increases. The Brinell hardness (HB) can reach around 50-60, which can better resist external scratches, wear and tear, and extend the service life of the steel.
However, if the thickness of the galvanized layer increases excessively beyond 200 microns, the hardness improvement effect will no longer be significant due to the properties of the zinc layer itself and the possible impurities and surface unevenness that may occur during the hot-dip galvanizing process. Even due to the influence of gravity caused by excessive thickness, stress concentration may occur at the joint between the zinc layer and the steel, affecting the adhesion of the zinc layer and adversely affecting the hardness and overall protective performance.
From these examples, it can be seen that increasing the coating thickness within a certain range can help improve hardness, but beyond a certain limit, due to various potential problems, the hardness may not only be difficult to continue to improve, but may also decrease, affecting the overall performance of the coating.

Room termperature curing polysilazane, pls check IOTA 9150, IOTA 9150K. 
High termperature curing polysilazane, pls check IOTA 9108, IOTA 9118.