HVOF Spray Coating Problem Analysis

High Velocity Oxygen Fuel (HVOF) coating is a Thermal Spray Coating process, which is used to improve or restore a component's surface (properties or geometry). This surface engineering technique better enables life extension of equipment by increasing erosion and wear resistance, and corrosion protection.

there are some problems on coating when spray by HVOF as bleow:

Coating bond fails at the surface

Reason1: surface contaminant(oil, water, variant matter)

Solution: keep parts clean, remove oil and preheat to 250°F(107℃)

Reason2:Incorrect preparation of substrate suface

Solution: Improve prepared surface profile by sandblasting or adding more threads for per inch(min=200-250Ra)

Reason3: Substrate overheat during spraying

Solution: Ensure proper spray distance, lateral speed and RPM

Reason4:geometrical shape of parts

Solution:the coating covers the edges, spray thinner on edges with less substrate

Reason5:Stress caused by coating overheating

Solution: 1. the spray is too thick for substrate geometry. to reduce the thickness.

2. substrate overheating, do not exceed 300℉(150℃)

3.coating is too hard, adjust spraying parameters to reduce heat. follow the instructions in the spray parameter table.

Reason6:spray angle of gun is not optimized

Solution:ensure that spray angle is optimal for maximum bond strength(90° is not always optimal-usually causes particles to bounce)

Coating cracks

Reason1: substrate overheating

Solution: cool down the substrate, do not exceed 300℉(150℃)

Reason2: coaitng is too hard

Solution: adjust spraying parameters to reduce heat, keep the substrate cool, improve the lateral speed and RPM

Reason3:powder feeding problem

Solution: 1. feeding rate is too low, so that the particles overheat, improve the feeding rate

2. carrier gas pressure or flow rate is incorrect. check against recommended parameters

Reason4: powder particles or existing coatings overheat

Solution: adjusting spray parameters to reduce heat, keep the substrate cooler, and increase the lateral speed and/or speed RPM, contribute to keep coating cooler and cool the parts more

Stratification of powder particles

Reason1:bond failure due to interlayer contaminants

Solution:ensure that dust particles are always moving from the substrate to all parts. dust removal may not be sufficient to remove oversprayed particles

Reason2:coating get too hot between each spray

Solution:adjust spraying parameters to reduce heat, keep the substrate cool, improve the lateral speed and RPM

Reason3:flame temperature is low and the particles are not sufficiently heated

solution:adjust spraying parameter to icnrease heat, increase barrel length or reduce pwoder feeding rate

Reason4:powder particle size is too large and particels are not sufficiently heated

Solution:increase the barrel length, adjust the spray parameters to increase the flame temperature, or reduce the powder feeding rate. use powder of suitable size designed for JP5000

Reason5:powder particle size is too small and the particles are overheated

Solution:reduce barrel length and increase powder feeding rate. use powder of suitable size designed for JP5000

High Porosity

Reason1: to pull out in turing, grinding or polishing

Solution: 1. going out with too much materials at once. one cut or grind is too rough

2.unmelted particles in coating, increase flame temperature to increase particle heat

Reason2:powder contains too many coarse particles

Solution:increase barrel length, increase flame temperature. use powder of suitable size designed for JP5000

Reason3:powder feeding rate is too high

Solution:reduce powdre feeding rate

Reason4:spray parameter is too low

Solution:adjusting the fuel/oxygen ratio to increase the flame speed gives more functionality

Reason5:angle of the spray gun on the rotating part

Solution:spray at an angle slightly inclined to the rotation direction of the part so that the spray direction is not 90° from the center of the part

Reason6:incorrect spray distance may be too far

Solution:move the spray gun to the correct spray distance, about 15[(38cm)

High oxide levels

Reason1:substrate overheating

Solution:reduce spray parameter to reduce heat, keep substrate cool, increase lateral speed and/or speed RPM, and check spray distance

Reason2:too much fine powders

Solution: 1. reduce barrel length and adjust spraying parameters to reduce flame temperature or increase powder feeding rate

2. use powder of suitable size designed for JP5000

insufficient hardnees of coating

Reason1:intergranular stratification

Solution: 1. adjusting the fuel/oxygen ratio to increase the flame speed gives more functionality

2. reduce powder feeding rate and increase particle heat

Reason2:powder particle velocity is too low

Solution:adjusting the fuel/oxygen ratio to increase the flame speed gives more functionality

Reason3:powder particle size is too large

Solution:increase the barrel length, adjust the spray parameters to increase the flame temperature, or reduce the powder feeding rate. use powder of suitable size designed for JP5000

Reason4:incorrect spray distance may be too far

Solution:move the spray gun to the correct spray distance, about 15[(38cm)

coating is too hard

Reason1:substrate overheating

Solution:adjust spray parameters to reduce heat, keep substrate cool, and increase lateral speed and/or speed RPM.

Reason2:high oxide levels

Solution:1. increase the lateral speed/RPM to keep the substrate cool or increase the powder feeding rate to reduce particle heat

2. too much fine powder, use powder of suitable size designed for JP5000

low deposition efficiency

Reason1:fuel/oxygen ratio is incorrect

Solution:if the particle rebound, adjust the spraying parameters to increase the heat of particles, pay attention not to overheat the substrate

Reason2:powder particle size is incorrect

Solution:use powder of suitable size designed for JP5000. too large powder particles cause rebound, and too much fine powder causes the flame to blow away the fine powder

Reason3:barrel length is incorrect

Solution:increase barrel length or increase flame temperature

Reason4:carrier gas parameter is incorrect

Solution:carrier gas pressure or flow rate is incorrect. check against recommended parameters

Reason5:coating too hard(see section coating too hard)

Solution:1. adjust spray parameters to reduce heat, keep substrate cool, and increase lateral speed and/or speed RPM.

2. increase the lateral speed/RPM to keep the substrate cool or inrease the powder feeding rate to reduce particle heat

Reason6:spray distance is incorrect

Solution:move the gun to the correct spray distance, about 15" (38cm), increase the spray distance in small increments and re-evalate the hardness and coating structure.